If you see errors like this:
Program version 4.6 doesn't match environment version 4.5 DB_ENV->open: DB_VERSION_MISMATCH: Database environment version mismatch
after an upgrade (to Fedora 8 in my case) it’s just that the old database cache files need to be removed. BDB will re-generate them.
In my case I did:
locate __db. | grep -v snapshots | xargs rm
which deleted all the caches that weren’t in my backups. Obviously, know what you’re piping to rm!