There might be more files, but I dont have a complete list of all files at hand. If you choose to do a backup before upgrdae in the installeer, then remove the while /usr/local/ispconfug directory and restore it from backup in /var/backups/.... plus restore the mysql dump.
Btw. Fixing your login issie in 3.0.5 RC would have been much easier. It should have been obvious that you cant downgrade a softawre without reinstalling, this is not possible with any complex software.