Invalid objects after a downgrade

First off, I'm only trialling this process on a test box, so there is no impact, and any parts of the process can be retried. I'm trying to upgrade a 12.1.0.2 SE2 database to 19c SE2. The upgrade process went fine, but as extra insurance I was looking to downgrade the database back to see if it worked. Part of the reason for doing this, is the vendors don't currently support 19c, but in it's current state, I can't patch it. It's been running fine on 12.1.0.2 (which is also unsupported by the vendors) for a few years, so