You cannot restore a backup from 7.3.8 and restore to an 8.0.8.1. You will have to upgrade the 7.3.8 server to 8.0.8.1 first and then backup and restore if that is a goal. There are major changes to database structure, etc where the upgrade has to be run to make changes to the database.
Additionally, there would be multiple upgrade steps as only 2 revs per upgrade are supported:
1. 7.3.8 to 7.5.7
2. 7.5.7 to 7.7.14
3. 7.7.14 to 8.0.8.1
Your other option is to just start a completely new server, archive the backup, and if you need the old data for some reason, restore it to an old AMP for investigations. Depending on size of server and number of devices, the upgrades might not be too long. Once a day is safe even for the largest servers, which at the end of the day is not too cumbersome of an upgrade, but waiting that long between upgrades just delays the inevitable pain :)
Good luck!