Changing Which LrC Catalog Syncs with Lr/Cloud

This is part 2 of a 3 part series concerning the sync process between Lightroom Classic (LrC), and Lightroom (Lr or Lr/Cloud).

Part 1 LR/Classic and LR/cloud Sync behavior discusses how various operations and commands done in Lr/Cloud and Lr/Classic affect synced images. 

Part 2 (this part) concerns the reconciliation process between Lr/Cloud and LrC when the synced catalog is changed and many of the problems that come from that process.

Part 3 Fixing Sync Problems caused by switching the Sync Catalog discusses options for fixing Lightroom Sync problems caused by changing the sync catalog.  This includes the dreaded “images don’t sync and can’t be made to sync” problem plus many others.

This article current as of LrC 14.0.1 and LR/Cloud desktop 8.0 (October 2024).  Versions of LrC or Lr/Cloud prior or after these may behave differently. I started my testing in August 2024 with LrC/13.3 but due to weird and inconsistent results waited for LrC/14 to complete the testing.

I apologize for the length of this article, I had not intended it to be this long but once I got into the testing, I kept discovering different use cases that produced different results.  I also found situations where running the same test on different days produced different results requiring more test runs to see if I had made an error in testing or Adobe was not being consistent.  All in all I probably ran over 40 full regression tests, each one consisting of 50 or more discrete steps using up to 100 test images in each test run.  Each test run spanned several days to several weeks depending on the complexity of the test. 

Many folks assume they know what changing the sync catalog does or just trust that it does what it should and don’t really think about what that means.  Most folks don’t really know what it will do and are quite surprised by what it does as many times it is not what they expect.  Therefore it is important that you fully (and I mean fully) understand what changing the sync catalog does and doesn’t do – it may not be what you think - especially in the more complicated scenarios. 

Unfortunately this is not something you can try on a few images first – it is all or nothing.  Nor can you ‘undo’ once it’s done in any automated fashion.  But, sometimes you just need to do it anyway and clean up afterward.

Why change the LR sync catalog?

Other than when you install a new version of LrC that includes a catalog update, normally one would not need to change which catalog is being synced with Lr/Cloud, but sometimes things happen that make this a best option.

I have identified 4 use cases where this is or may be necessary.  I’m sure there are more.

I will be talking about these four cases individually as each tends to produce different results in some situations.  For each one I’ll discuss what happens depending on the contents of the old and new catalogs and what is in Lr/Cloud.  Where appropriate I’ll also give some tips on how to deal with some of the undesirable behavior you find when switching the sync catalog in some cases but also see part 3 of this series: Fixing Sync Problems caused by switching the Sync Catalog.
.

Here are the 4 use cases I discuss in this article. 

1. Upgrading LrC to a new version which requires a Catalog Update.
2. Revert to a backup catalog where you have not made any LrC or Lr/Cloud changes since the BU was created.  An example is that the disk drive where your catalog lives failed overnight and can no longer be used but you have a backup from just before you went to bed.
3. Revert to a backup catalog from a while ago where you have made changes in LrC and/or Lr/Cloud since that backup was created.  For example you’ve been negligent in taking regular backups and your master catalog failed or was lost and you have to go back to a catalog from days, weeks or months ago.  Another example is if you find some corruption or you made a big mistake in the catalog several weeks or months ago and you need to revert to a catalog from before when that occurred (although importing from that older catalog may be a better option).
4. Revert to an unrelated catalog.  For example you created a new, empty, catalog and re-imported all your images where none, some, or all had been in the previously synced catalog and may or may not have been synced to Lr/Cloud.  Another example is switching to a catalog that was created using the “Export as Catalog" function.

I think of cases 1, 2, and 3 as switching to a “blood relative” catalog of the previously synced catalog.  By this I mean the new catalog was created as a copy of the previously synced catalog.  On the other hand case 4 is switching to a non blood relative of the currently synced catalog.  This does matter.  In the former case the new catalog retains the same internal catalog ID and image ID’s as the previous catalog.  In the later case the new catalog has a different internal catalog ID and different image ID numbers and things happen differently.

I’m sure there are other situations which require you to switch the sync catalog but they would probably fall into one of these 4 cases.  Others I have thought of are near the bottom of this article.

What happens when you change the sync catalog varies quite a bit depending on what’s in the new catalog and how it got there,the differences between the currently synced catalog and the new catalog you want to be the synced catalog, and the content of your Adobe Cloud account.

NOTE: When you are syncing, most of the actions performed in LrC or Lr/Cloud on synced images require information to be “synced” to the other ecosystem.  This is not instantaneous as it involves passing things through the internet and processing at both ends.  The time required to do this changes depending on how many images are involved, what information must be passed back and forth, and your internet speed.  So, sometimes being patient is required.

Which catalog is syncing

When you open an LrC catalog where sync was previously turned on or you un-pause sync in your open catalog,  LrC checks to see if the open LrC catalog is the ‘current sync catalog’, is a ‘blood relative’ of the currently synced catalog, or is an ‘unrelated’ catalog.  Depending on which it is different things happen.

DEFINITION: A Blood Relative (my term) of the currently synced catalog include the following:

• A backup catalog taken by LrC upon exiting
• A catalog backup you created yourself
• A new catalog generated through the LrC upgrade process

It should be noted that a catalog created using “Export as Catalog” or “Create New Catalog” followed by “Import from another catalog” are NOT considered a blood relative of the source catalog.

To change which catalog is syncing, in LrC click the cloud icon in the far upper right corner.  In the drop down it will either show that the sync process is paused with a button to “Resume Sync” or it will show that it is active with a button to “Pause Sync”.

Sync Active (nothing in process)
01 LrC - Sync Active

Sync Paused
02 LrC - Sync Paused

If sync is active, by definition the catalog you have open is the “currently synced catalog”.  If it is paused and you click the button to start syncing and the catalog you are in is the currently synced catalog (or a blood relative of the currently synced catalog) it will just resume syncing.  But, if the catalog you are in us not a blood relative of the currently synced catalog you will get a dialog explaining this and it will ask if you want to start syncing this catalog instead.

Change sync catalog approval dialog
02 Intro Sync unrelated catalog approval

Terminology

In the text of this article I use various terms and abbreviations and it is best that I go over them as some are my own terms.

Sync Catalog

The “Sync Catalog” (or “Currently Synced Catalog”) is the LrC catalog that is set to sync to your Lr/Cloud account.  Only one LrC catalog can be set to sync with your Lr/Cloud Lightroom system at a time.

XFER Folder

I will be using “XFER Folder” in this article to mean the destination folder where LrC places images downloaded from Lr/Cloud.   By default this is “Mobile Downloads.lrdata” in the “Pictures -> Lightroom” folder (on a Mac this is a “package”) but Adobe tends to change this from time to time. 

Most people use the “Lightroom Sync” tab of the “Preferences” dialog in LrC to change this to a regular folder in their system.  In either case you can elect to have all the images downloaded from Lr/Cloud go into this one folder or have LrC automatically create subfolders under this folder by capture date using a pattern you select (same choice you have in the Import dialog).  For my testing I did not choose “Use.subfolders formatted by capture date” but I have no reason to believe it would change the findings if I had.

01 LrC - XFER folder setting

DSP folder (“downloaded-smart-previews”)

I use “DSP folder” to mean a folder named “downloaded-smart-previews” which is created by the sync process as a subfolder under the XFER folder in some cases which we’ll get into.

Original Image

Original images are those that entered the LR world through that LR ecosystem.  If an image was imported or taken with one of the Lr/Cloud apps then in Lr/Cloud it is considered an original.  Conversely LrC original images are those imported directly into LrC.

SP’s

These are Smart Previews.  An SP is a proxy image that is sent to Lr/Cloud from LrC when the image is set to sync.  They are smaller version of the original files and are 2550 pixels on the long edge and can be used in Lr/Cloud for editing and review.

01 Case 3 - SP's

VC’s

These are Virtual Copies within LrC (there is no equivalent in Lr/Cloud).  A VC in LrC is an entry in the catalog which for all intents and purposes looks like and behaves like a standalone image but in fact it points back to the same master image file on disk as some other image that is in the catalog.  In other words it is a copy solely within LrC that can have a different treatment than the master file.

CAT1

This is the currently synced catalog before you tell LrC to make CAT2 the synced catalog

CAT2

This is the new catalog that you want make the synced catalog

Principles of the Adobe Sync processing

It should be noted that in some cases these principles are somewhat theoretical as in actual behavior at times they are not adhered to.

1. Only 1 LrC catalog can be synced to the cloud at a time.  This is called the “Sync Catalog” (or “Currently Synced Catalog”)
    
2. Every image in Lr/Cloud is synced to LrC
   There are exceptions caused by switching the sync catalog which I consider a bug but which Adobe said is  “as designed” in at least one occasion.
3. Videos added to Lr/Cloud will be copied to LrC once.  After that they will not sync.
4. Lr/Cloud Original images, will sync full size original copies of the image (in its imported file type) to LrC which are added to your file system but the “original” is still considered as being the version in Lr/Cloud and those originals are counted against your plan’s storage limit.  In LrC these images are placed in the designated XFER folder as well as the “All Synced Photographs” special collection in the Catalog Panel.  If the image was in one or more albums in Lr/Cloud, the image will also be placed in corresponding synced collections in LrC.
5. In LrC, only images in the “All Synced Photographs” special collection in the Catalog Panel will sync to Lr/Cloud and this is restricted to still images only (no videos).
   02 All Synced in Catalog Panel
   
   However after switching the sync catalog there may be cases where images in the “All Synced Photographs” special collection or in other synced collections are not present in or synced to the Lr/Cloud.  I consider this a bug but Adobe says it’s “as designed”.
6. When LrC original images are synced to Lr/Cloud, a proxy image called a Smart Preview (SP) is what is sent to Lr/Cloud.  This is a file with 2550 pixels on the long edge that stands in for the master copy of the image and can be used for editing.  These do not count against your plan’s storage limit.
7. Where synced images appear in both Lr/Cloud and LrC with a “sync” relationship, changes to the image in either ecosystem syncs to the associated image in the other ecosystem within the limits of “What does and doesn’t sync” (see link at bottom of this article).
   
   After changing which catalog is the sync catalog (for example after reverting to a backup catalog), there are several scenarios where images in LrC are shown as syncing but there is no matching image in Lr/Cloud and the copy in LrC cannot be made to sync.  There are other scenarios where the images in Lr/Cloud show different edits than its mate in LrC.  Subsequent changes with controls not previously synced do sync but changes to edits previously synced do not sync to the other ecosystem

A way of thinking about what happens

This may not be an accurate description of how it is actually implemented but is a good way to think about how it operates (an analogy if you will).

1. An LrC catalog has an internal ID (let’s call this the “Catalog ID”).  This number is in the catalog itself.  If you backup, or use your OS to physically copy a catalog the number stays the same.  However, if you create a new catalog or use the “Export as Catalog”, you get a new catalog with a new Catalog ID.
2. Each image in LrC and Lr/Cloud has an “Image ID” number.  The same image imported into two different catalogs may have different Image ID numbers.  This includes those in a new “Export as Catalog” catalog.
3. The Catalog ID plus the Image ID is how the sync process matches images in Lr/Cloud with their mate in LrC.  In other words for there to be a sync relationship the numbers must match
4. Due to this, if you change the sync catalog to one with a different Catalog ID, then no image in Lr/Cloud will match to any image in the new catalog and every single one will be copied form Lr/Cloud to LrC as if it were a new image imported into Lr/Cloud.
5. Similarly any image in LrC that you set to sync will be uploaded to Lr/Cloud as a new image.
6. As I’ll discuss below, it is quite easy to find yourself in situations where the ID of a synced image in LrC does not find an image in Lr/Cloud with that ID number – and vice-versa.   It seems that the sync architecture was either not designed to handle these situations or was incorrectly coded and the result, as I’ll describe below, is a collection symptoms that I consider bugs.
   
   For corrective measures to these problems see part 3 of this series of blogs Fixing Sync Problems caused by switching the Sync Catalog

Case 1 - Upgrade to a new version of LrC

When you upgrade LrC to a new version of LrC, in some cases (e.g., LrC/13.5 to LrC14.0.1) this involves an update to the structure of the LrC catalog.  When this happens, a copy of the current catalog is created and updated and this new version becomes the synced catalog and is ca blood relative of the catalog it was created from.  In this case the sync process just switches to the upgraded catalog and picks up where it left off from the prior catalog.  I did not find any issues with this case.

04 Case 1 Catalog Update Reaquired

Case 2 - Revert to a CURRENT BU of the synced catalog

Case 2 is when you revert to a catalog BU and make it the synced catalog where no changes have been made to synced images in LrC or in Lr/Cloud since the catalog backup was created.

For example, you created a BU of a fully synced and running fine catalog but that catalog becomes unusable before you’ve made any additional changes to synced images in either LrC or LR/Cloud and to fix this you revert to the BU catalog.

When you open the BU up catalog, if sync had been on when the catalog backup was created, the sync starts automatically.  Otherwise sync starts when you un-pause it.  When sync starts, LrC automatically (and without any popup or message) performs a reconciliation of what’s in the recovered LrC catalog and Lr/Cloud.  In this case, there is nothing to reconcile so it just picks up syncing where the previous catalog left off.  I found no issues with this case. 

If you had made changes to UN-SYNCED images in LrC after the backup was created, those changes will be reverted to their state in the BU catalog.  If you had saved those changes to XMP you can recover them to the state at the time the XMP was saved to disk by “reading metadata from disk”.  However if you had made any changes to synced images in LrC since the backup or any images in Lr/Cloud since the backup you need to look at case 3 below.

Case 3 – Revert to an OLDER BU of the synced catalog

Case 3 is where we start getting into complexity and is the most complex of the cases I describe in this article.  This is when you revert to a catalog BU where you’ve made changes to synced images in either LrC or Lr/Cloud after the backup was made but before making the BU catalog the new synced catalog.  This is typically the case when you have to revert to an older catalog backup because more recent catalog backups exhibit the same problem that you’re trying to get rid of or you just don’t have any current catalog backups.  Depending on your problem, you may want to consider an “Export as catalog” from the BU catalog and then “Import from Another Catalog” to bring that info into the currently synced catalog instead.  Which you choose depends your particular situation.

05 Case 3 Older BU

Like case 2, if sync was on when the catalog backup was created, then sync starts automatically when you launch the recovered catalog.  Otherwise sync starts when you un-pause sync.  When sync starts LrC automatically (and without any popup or message) performs a reconciliation of what is in the recovered LrC catalog and Lr/Cloud.  In this case the reconciliation can get quite complicated depending on what you’ve done in LrC and/or Lr/Cloud since the backup was taken including new imports, edits, deletes, removals, and moving or renaming files or folders.

Case 3 Summary of what works OK

• Un-synced images are restored as they were in the BU Catalog.
• Synced LrC originals not edited since the BU was created are restored as they were in the BU catalog
• Synced LrC originals edited since the BU was taken come back with the restore but are then updated to the edited version stored in Lr/Cloud
• Synced Lr/Cloud originals in the BU catalog which were not edited since the BU was created.  The full size files previously synced down to LrC from Lr/Cloud come back with the restore.
• Synced Lr/Cloud originals edited in LrC and/or Lr/Cloud after the BU was created.  The latest version from either LrC or Lr/Cloud is placed in both LrC and Lr/Cloud.
• Images imported into Lr/Cloud after the BU was created (and therefore not in the BU) sync back to LrC as full originals and are placed in the XFER folder.

Case 3 Summary of what doesn’t work quite as well or could be confusing

• Mixed behavior.  Un-synced images are restored in LrC as they are in the BU catalog but synced images retain edits done in either LrC or Lr/Cloud after the BU was created.
• Smart Previews invade LrC Catalog.  Images imported into LrC and synced to Lr/Cloud as SP’s after the BU was created are present in Lr/Cloud but not present in the recovered catalog.  These SP’s from Lr/Cloud are downloaded to LrC.  In LrC they show up in the DSP folder which is a subfolder of the XFER folder.  However the image is only in the LrC catalog, it is not created on disk even though the DSP folder is created on disk.  These images are considered as missing but with an SP in the catalog.  Placing an original image file in the DSP folder using Finder/Explorer makes it no longer missing.
  07 Case 3 SP's Invade Catalog
  
  What’s unknown is whether this is the desired behavior or not.  I can see situations where this is desired and other situations where it is undesirable.  For example, if the purpose of going to a BU catalog was to “undo” everything done recently then this is not desirable but if the purpose was to fix catalog corruption then this may be desirable as those SP’s have all the recent updates.
• Empty Cells.  These are empty grey cells in the gird/filmstrip.  Even in normal operations this can occur when images are “missing” and there is no preview in the catalog.  But changing the sync catalog can also cause missing images to appear in the catalog.
• Orphan images in LrC.  An ‘orphan’ image is where the image is in LrC, has the icon showing that it is a synced image, appears in the “All Synced Photographs” special collection and may also appear in one or more other synced collections but is not in Lr/Cloud and will not sync to Lr/Cloud. 
  
  This occurs when images are present and synced at the time the BU was taken but were subsequently removed from LrC or Lr before the BU was restored and then come back into LrC with the restored catalog.  This also results in the counts of synced images to be different between LrC and Lr/Cloud.
• SP’s and VC’s.  After making the BU catalog the syncing catalog, sometimes  SP’s (Smart Previews) will sync down to LrC and sometimes VC’s (Virtual Copies) will sync down. 
  
  You’ll get a VC when you have edited but then deleted a synced image after the BU was taken then after the restore you un-delete the image in Lr/Cloud.  The un-deleted version comes down to LrC as a VC of original image restored from the BU catalog.
  
  You’ll get an SP when you import an image into LrC after the BU was created and sync it causing an SP to sync up to Lr/Cloud.  When the BU catalog is restored and made to be the sync catalog that original image is no longer present, but the SP in Lr/Cloud is still there.  That SP in Lr/Cloud then syncs down to LrC as an SP.  You’ll also get an SP when you remove a synced LrC original after the BU is created and then un-delete it in Lr/Cloud after the BU catalog becomes the synced catalog
• Different edits in LrC and Lr/Cloud.  When you restore the BU catalog, images appear in LrC as they were when the BU was created.  If you had edited images in both Lr/Cloud and LrC after the BU was created all those edits remain in Lr/Cloud but are not in the restored catalog.  What happens to those images that came back with the catalog restore depends on where the last edit was done! 
  
  If the latest edit was done in LrC, the image in LrC losses both edits (retains the state from the restored catalog) but as the version in Lr/Cloud retains the latest edits there is a mismatch between what is in the cloud and what is in LrC.
  
  Top screen shot shows an image in LrC which lost the edits and bottom shows the same image in Lr/Desktop which retained the edtis
  06 Case 3 Post BU edit mismatch
  
  In this case further edits done in Lr or LrC sync, but ONLY edits done with different controls.  Further edits to the same controls that were undone by the catalog restore do not sync to the other ecosystem even if they are changed again. 
  
  If the latest edit had been done in Lr/Cloud the edit from Lr/Cloud is synced to LrC correctly and there is no mismatch.
  
  NOTE:  This result occurred in 3 test runs in November but in December (using the same versions of LrC and Lr/cloud) these images all retained their most recent edits in two successive tests.  I am leaving this in this article as you may experience this behavior even though I can no longer re-produce the problem. 

Case 3 More details

As this case is the most complicated,  You can find more details on the various situations involved in this addendum article.  I should note that the addendum article is really just notes I made while testing to help me categorize and group the results.  I cleaned up the presentation a bit but this is still just notes and should be considered as such.

Changing to a non blood relative catalog

When you turn on sync in such a catalog you will get this warning dialog.

02 Intro Sync unrelated catalog approval

If you select “Yes, sync this catalog instead”, LrC and LR/Cloud will stop syncing the old catalog and will start syncing this one and will commence on a reconciliation process.

HINT 1: In LrC, just prior to re-assigning the sync catalog, either move all the images out of the designated XFER folder to other folders or create a new folder and designate it as the XFER folder.  In this way, you will be able to find any newly downloaded images from LR/Cloud as a result of syncing the new catalog.  Believe me, you will be happy you did this and even if it wasn’t necessary, it’s best to be on the safe side.

HINT 2: Depending on your situation, it may be worthwhile, with sync paused, to remove all images from Lr/Cloud before you swap the sync catalog.  There are many factors which should be considered before doing this but if your plan is to “start over” with a new catalog and you would prefer that everything in the cloud gets erased, this may be something to consider.  However you should realize that anything that is ONLY in the cloud like un-synced edits or un-synced Lr/Cloud original images will be lost.  Also be aware if you take this approach that ONLY things in the new catalog will sync to the cloud and only if they are not ‘missing’ AND you tell them to sync by placing them in a Synced Collection or the ‘All Synced Photos” special collection.

Case 4 – Sync an unrelated catalog

This is where you change the sync catalog to one that is not a blood relative of the currently synced catalog.  This includes:

• Changing to a catalog created from an “Export as Catalog”.
• Changing to a new catalog where you have re-imported some or all the images from the previously synced catalog with our without the addition of other new images
• Changing to a catalog with no relation to the previously synced catalog

Note:

My initial assumption was that this use case would be the most complicated given the complexities of case 3.  However, it turned out that this use case was simpler to understand even though the results can be dramatic.

Sync an “Export as Catalog” catalog

This includes when you want to create a new catalog (CAT2) using the “Export as Catalog” function from your master catalog (CAT1) and then have the new exported catalog (CAT2) become the catalog that syncs with Lr/Cloud.  Typically this new catalog would contain all the images from the previously synced master catalog.  Even though this new catalog is derived from the previous master catalog it is not considered as a blood relative catalog.

03 Case 4 - Export as Catalog menu

It should be noted that in the exported catalog (CAT2), nothing is initially set to sync and sync is set to “Paused”.  Your collections will be present and populated but will not be marked to sync and the “All Synced Photos” special collection will be empty.

Case 4 Results

Albums and Synced Collections

• Albums already in Lr/Cloud remain unchanged
• All Lr/Cloud Albums will be replicated in the new LrC catalog as NEW synced collections and will be placed under collection set “From Lightroom”.  This is done for ALL albums in Lr/Cloud regardless of whether or not collections with the same name already exist in the new catalog.  If one with the same name already exists in the “From Lightroom” collection set, the new one downloaded from Lr/Cloud gets a “-2” appended to the collection name.  This may result in 2 versions of each such collection if one was already there which would be the case with an “export as catalog” situaiton.
• If you subsequently mark any other LrC collections to sync in CAT2, they will become NEW Albums in Lr/Cloud.  These too could be duplicate albums especially in the “Export as Catalog” case.

Images

• Un-synced images in the new catalog remain un-altered by making it the sync catalog
• Images already in LR/Cloud remain un-changed in Lr/Cloud.
• All images and video’s in Lr/Cloud at the point where you make the new catalog the sync catalog will be downloaded and added to the newly synced catalog as new entries.  All (except video’s) will be marked to sync and will be placed in the “All Synced Photos” special collection in the Catalog Panel.  This is done for ALL images in Lr/Cloud regardless of whether or not the new catalog already has a copy of those same images in the catalog.
• For images in Lr/Cloud that are full size original files (originally imported into Lr/Cloud), the un-edited full size original file, in its original format, is downloaded and placed on disk in the XFER folder.  A new entry is added to the catalog for each of these images containg the edits from Lr/Cloud but the history panel will only show “From Lr Mobile”.  If the new catalog already has a copy of the image, you now have two in LrC and two on disk.
• Where images in Lr/Cloud are SP’s (originally uploaded from LrC), the SP’s are added to the LrC Catalog as new entries and are shown in the Folders Panel as being in the DSP subfolder under the XFER folder.  No physical image file is added to your file system.  The DSP folder is created on disk if not already there, but images are not placed in it.  In other words the images are “missing” according to LrC and have only an SP.  If you subsequently place a copy of the image in the DSP folder on disk it will no longer be ‘missing’.  But if you no longer have an original in your system, the SP is as good as it gets and you’ll only have the SP (it is possible to create a physical Jpg from the SP but that’s another subject).  If the new catalog already had a copy of the image, you now have two in LrC. In the catalog the SP will contain the edits from Lr/Cloud but the history panel will only show “From Lr Mobile”
• For both SP’s and Full files, if the XFER folder on disk already contained a copy of the image file, the file name of the newly downloaded version get’s a “-2” appended to the end of the file name.
• If you subsequently sync any images in the new catalog, those images will be synced to Lr/Cloud as new images and sent to Lr/Cloud as SP’s whether or not they are already in Lr/Cloud. 

Case 4 - Suggestions & Steps

As you see from the above, images in Lr/Cloud (be they Lr/Cloud full size originals, or SP’s synced up from LrC) sync down to CAT2 as new images – duplicating images already in CAT2.  And images in CAT2 that had been syncing in CAT1 are no longer syncing. 

For the Lr/Cloud originals (full size images in Lr/Cloud), you can either delete the version in Lr/Cloud and re-sync it’s mate in CAT2 as an SP or you can remove the previously synced version in CAT2 and let a new one from Lr/Cloud sync down to CAT2.

The bigger problem comes from images that originated in LrC and had synced to Lr/Cloud as SP’s.  Here are some ideas to avoid many of the Case 4 problems but also see Fixing Problems Caused by Changing the Sync Catalog on how to fix those problems if they have already occurred.

If you are starting a new catalog (rather than an “export as catalog” catalog),  you may want to first clear everything from Lr/Cloud.  This is discussed in the URL listed in the previous paragraph.

If you are switching to an “Export As Catalog” catalog then:

1. Before you do the export as catalog, mark (or make a list of) all your synced collections (you can use a color label on those collections for this).  This will help you find them again after they become un-synced in CAT2.
2. Before doing the ‘Export as Catalog’ make a note of which synced collections have a shared URL (make public) if you will be re-sharing them after you change the sync catalog.
3. Copy your “All synced photos” special collection into a regular (un-synced) collection
4. Then do the “Export as catalog” to create CAT2
5. Pause Syncing in CAT1
6. Close LrC
7. In Lr/Cloud, delete all the images that are SP’s.  There is a filter for this in Lr/Desktop
   03 LR filter for SP's
   but leave the albums they had been in.
8. In the exported catalog (CAT2), before turning on sync, set the sync folder (preferences -> Lightroom sync) to an empty folder so that all the items downloaded from Lr when you set it to be the sync catalog don’t get intermingled with images previously downloaded from Lr/Cloud.
9. In CAT2, before turning on sync, if you have any collections in the “From Lightroom” collection set, move those collections to some other collection set. We do this so that albums downloaded to LrC as synced collections due to the catalog change will not be intermingled with previous collections.
10. In Lr/Cloud delete any empty Albums to keep them from replicating in LrC
11. Set CAT2 to be the sync catalog
12. For each collection that is created in the “From Lightroom” collection set, drag the images from your original collection with the same name into it.  These will now sync up to Lr/Cloud as SP’s replacing the ones you deleted in a step above.  This will also preserve the shared URL if any of these Collections/Albums had been shared.
    
    04 Case 4 - Collections Panel
13. At this point you can delete the original (no longer synced) collections that came into CAT2 from the CAT1 export as catalog

Case’s not investigated

• Variant of case 4 where the NEW catalog had previously been the sync catalog at some point in the past.  For example you are syncing CAT1 and for some reason set CAT2 to be the sync catalog.  After realizing the mistake you go back and set CAT1 to be the sync catalog again.  I suspect that this scenario will be the same as case 4 twice – once going from CAT1 to CAT2 and then again going from CAT2 to CAT1.
• Taking a catalog that was synced to the Adobe Cloud under a different Adobe User account and then making it the synced catalog for this Adobe Account.  I assume this is no different than case 4.
• In case 4 when switching to a current “export as catalog” catalog.  I did not test where the exported catalog was older and updates had continued to be done after the export but before swapping the sync catalog.  I choose not to include this as I could not envision a set of circumstances were this would happen in real life.  I assume the user would just do the export as catalog again if a bunch of changes had taken place since the prior export as catalog.

What does and doesn’t sync

Please see this article on Lightroom Queen Website

https://www.lightroomqueen.com/limitations-syncing-classic-with-cloud/