Cannot upgrade Lightroom Classic Catalog

Hi, I’ve been researching for a while and tried a few options and not sure how to raise a ticket with Adobe directly so really hoping someone in the community can help please!

I upgraded Lightroom Classic to 15.5 and couldn’t upgrade my catalog with it, it kept failing. I had synced to the cloud to edit on my iPad so I started to continue editing in Lightroom CC and forgot about it until I came to needing to export all my original images (so unfortunately I can’t remember what version I was on before or exactly what the error was as I did this in July) but all I could export on LCC today was my Smart Previews and I need to export my full size files to deliver to a client. 

I’m on Windows 10 version 22H2

I’ve tried to open my catalog again in an older Lightroom Classic 15 and downgraded to 14 but it fails when I click to upgrade the catalog. I’ve created and opened a new catalog in old and new versions and tried to import my old catalog but it fails again on upgrade. It says “Unable to upgrade and cannot import from it. Try opening in Lightroom 7.” Where can you get that version? The support pages don’t show that available for download for me.

I also tried opening while holding down Alt and going through the process again and checking the integrity. My catalog is stored on the local disk so I didn’t see the point in moving the catalog around as I saw suggested if the catalog was on an external HD. I think I have enough space for the upgrade on the disk as well, but I’m not sure how to check how big my library was and the catalog was linking to photos on Dropbox and other places. I have no way to open the old catalog to click Migrate or Export Catalog or do anything with it now.

There is no lock file or other side files or lrcat-data next to it. It also doesn’t say that the file is corrupt but I can’t seem to open with any version of Lightroom as it needs upgraded for every version.

And stupid me, I can’t find a backup file! I think I might have sync data files in folders if that can do anything?

I’d rather not lose the catalog but in order to solve this issue short term just to get the images to the client, I clicked Export with Original Settings to get the DNG files with the colour/exposure adjustments so that I can hopefully import it into a new Lightroom Classic catalog and relink the originals there… 

TLDR; so I’d be really grateful for any ideas on how to retrieve this LRC catalog and then upgrade it for use on Lightroom Classic 15.5.1 successfully please? Any thoughts welcome, thank you :)