I wanted to downgrade because the new licensing model didn't fit our needs and the procedure to buy extra licenses would take some time. Basically, we've applied the new licenses too quickly. Human error.
The biggest problem was that we didn't have IDB backup for version 8.0. Sadly, the problem wich backuping the database was reported months ago to support and they never solved the case. Our IDB was then upgraded to 8.1 together with the software.
I reinstalled a clean 8.0 enviroment and copied config files, which imported all the backup specs and clients. The old licences (lic.dat) worked as expected. Then I had to manually set up devices and schedules. I was unable to just replace the new and clean IDB with the previous one (filesystem copy), tried manually correcting the passwords and fixing the errors in logs but without success. Probably because the database was upgraded to 8.1.
We still have the old 8.0 DB (filesystem copy) on tapes which would be great if we could import to the new database (still 8.0), because it contains catalog for older tapes. Importing catalog directly from tapes is a very slow procedure and the stack of tapes isn't that small. Maybe someone has some suggestions?