How about making sure that once the backup has run on these tapes they are moved outside of any active POOLS so another backup does not try to write to them too (and fail).
You want drives to go down if it cannot write to a tape as any bad/faulty...
Just deinstall the 7.1 software and reinstall 7.6 - no point in "upgrading over the top" as we don't keep anything of worth in a clients NetBackup install.