Man, sorry for the lack of info - I got ahead of myself.
We use Sony 150P 20/40GB DDS4 tapes. Nothing fancy as far as our scheme/hardware, and we do a full backup every night. One tape/month is taken off premises permanantly. We have been using the tapes since 05/2003.
So, I was going to order 40 new tapes and change the backup scheme to a "Grandfather" scheme as Veritas notes in their manual; using differentials and so on. But the more I read, the more I see scarey scenarios with differentials. Hmmmm. What to do????