1. create backup in 8.2
2. unzip backup file
3. find info.xml
4. find element <psadump>
5. change attribute psaversion from 8.2 to 8.1.1
6. zip backup file
7. try to restore it on 8.1.1
I try not to uninstall 8.2 beccause last time I did it didn't finish completely, I had to reformat the machine. Errors (freezes of application pool) still occur, it is documented in this thread: