Moving to Lr from Lrc question

What's your thinking behind the plan to export the existing synced LrC catalog to a new catalog, then migrate that new catalog to Lightroom?

The logical thing to do would be to migrate the existing synced catalog, and doing that should indeed replace the existing smart previews with the originals in the cloud. Doing it your planned way it is entirely possible that you would end up with duplicates as the current synced smart previews are associated with the original synced catalog, not the newly migrated one. I don't know that for certain, as that's not a scenario that I ever tested, but it is a possibility. Furthermore, if after the migration you open the current synced LrC catalog, with sync still enabled, everything that you added to the cloud by migrating the exported LrC catalog would automatically download from the cloud into LrC, which would almost certainly create either duplicate files or Virtual Copies of those files. Unless you have a strong reason for doing it your way I'd keep things simple and migrate the current synced catalog.

I can not directly answer your question. However, I suggest following these steps:

With Classic, only SmartPreviews for the synchronized collections are uploaded and stored in the cloud.

If you deactivate synchronization for all collections, that should clear them from the cloud ... give LrC some time to sync.

To verify and to also clear the trash bin in the cloud, log in to https://lightroom.adobe.com and clear everything out that may still be there. Once all is cleared, deactivate syncing in LrC.

That should give you a clear start to move the images to Lr.

Please make to set up Lr correctly before starting to move photos, as by default every photo you import is stored to the cloud. If you want to work locally, make sure to verify the settings first.

Use the integrated migration option in Lr to move from LrC to Lr.