Indeed, we had to write a script to "suspend" tapes from each night's 
backup so they don't keep being written to.  Otherwise the current tape 
(for that retention period) was likely to be in use, so did not get 
offsited the next day.  If you have Vault, it is a built-in option, 
otherwise you have to script it.

This is a one-off task though - so I would not use too much time on it. 
NBU will know which tapes are expired and so reusable, do you really need 
to fill the empty slots with "expiring soon" tapes?

Otherwise I would suggest using just a list of expired media, even if you 
have to spend a week or two producing new lists until you hit steady 
state.

Big libraries are for people who don't offsite....because the library is 
already offsite.   The rest of us use lots of drives and White Van Man to 
offsite the tapes the next day.

William D L Brown