CU8 was just released. The Veritas proliferation commitment is 90 days to post support in the SCL.
You ask if you can back up anyway. Probably. NetBackup won't reject CU8 a priori. Database backup and restore is almost sure to work. I suggest you take a backup and try a restore to RDB.
GRT backup and restore works unless Microsoft changes the schema, as they did last spring with CU5. The only other time in the last 10 years that I can recall a schema change on an SP or CU that affected NetBackup was Exchange 2010 SP1.
You should move up to NetBackup 8.1 if you're going to use GRT. Otherwise, you need a patch for 7.7.3 or 8.0 due to the CU5 schema change.
Let us know how it goes.