I believe this article will answer your queries :
Since, you want to keep all your data, the option "Preserve old settings as a Virtual Copy" would be best for you.
And before merging both the catalogs, make sure you have backups for both of them separately.
Thanks, Chetna. will the virtual copies appear with each other, as usual for copies? I assume I can then compare and eliminate unwanted ones. Will this only make virtual copies ONLY for differing versions, or for each of my 56k files?
It will make virtual copies for those images which have been edited in the other catalog (for different versions only) and will show you the preview and the count of those images when you will go to that option to import from another catalog.
OK, thanks, I’ll try this in the next couple of days and report back.
I've merged the two catalogs, but it appears I've "lost" at least one collection. How do I get this back? And is there a way to sort/find all files whose metadata differs? (has the arrow icon in the corner of the thumbnail)
Merging would not affect collections. All the collections should be there in that merged catalog.
In order to filter for the images that have different metadata (these are called virtual copies) :
1. Select the folder under which you want to look for virtual copies, or you may select All Photographs under Catalog Panel.
2. click on Photo menu > Stacking > Expand all stacks
3. click on Library menu > Filter by Kind > Virtual Copies