I've just upgraded from Lightroom 3 to Lightroom 6. When I try opening or importing from an existing catalog that was working fine in Lightroom 3, Lightroom says the catalog has to be upgraded. When I opt to upgrade the catalog, the dialogue gets stuck (for several hours) at "Catalog Upgrade in Progress--Preparing New Searchable Database". If I opt to import from a catalog, it hangs at the 60% mark. It seems likely to be the same error in both cases.
Is there any alternative to reverting to Lightrom 3 and cancelling my subscription?
What operating system are you using and thew exact version of the OS, Mac or Windows?
Check that the folder the catalog is trying to use for the upgrade has both read & write permissions for your username. If on a Windows system that would be Full Control for your username.
Make sure the drive the catalog is on has enough space available for the copying of the LR 3 catalog to a LR 6 catalog.