I appreciate your replies
plandata_at thanks for your input; this is a very small implementation: a tape library with one tape drive and about a dozen VMs - so on that front I ought to be OK.
I need to read up on exactly what's involved with a GFS media pool but my (completely ignorant) initial reaction is that backups already taken under the current tape scheme using manual backup pools will be affected. I think I might need to get a bunch of tapes that have aged out their data to test having two concurrent backup pools and also that a GFS media pool will be able to exactly conform to the existing tape specification:
Weekly (Protect data for 4 Weeks)
Monthly (Protect data for 11 Months)
Quarterly (never overwrite data)