Hello,
The tape rotating concept under Veritas is called
"Retention Level". You set a retention level
chosen among the pre-defined ones when
you set the properties for your different schedules.

Retention levels can be defined in number of days,
weeks, months, or even years (I think). You access
the predefined retention levels under 'bpadm', then
you choose 'x' for Special Actions and then 'r' for
Retention Levels.

When Veritas uses a tape, it stores current date each
time it writes a new backup to it. Let's say you want
to rotate tapes each 3 months. Then you define a
retention level to 13 weeks, and use this retention
level in your schedules. When Veritas notices a tape
has not been written to for 13 weeks, then that tape
expires and becomes status "AVAILABLE" again in
'available_media' instead of "FULL" (or "ACTIVE").
The data is not erased from tape, but since that tape
has AVAILABLE status, Veritas is authorized to use
it for new backups and the old data is overwritten
when Veritas uses it for a backup. 

If you find out all your tapes in a specific pool are
"FULL", that means the retention levels you use in
your schedules are too long or set to infinity (the
default). You should then verify what retention level
your schedule uses and choose one that corresponds
to the frequency you want to rotate your tapes (or
maybe a little less to allow tapes to expire before
Veritas attemps to use them again).

If you already have "FULL" tapes you want to free
right away, you can expire it manually and the
status will be "AVAILABLE". Just type

# bpexpdate -ev <media id> -d 0

Good luck.
Louis-Luc