Re Andrew,
how exactly is your D2D integrated with Data Protector? VTL? Classic FileLibrary? DFMF FileLibrary? You cannot generally get rid of expired objects that share a medium (of any type), which is why file oriented media is often operated with no-append mode, so it doesn't accrete too many objects of different protections. But if you have such a situation, the easiest thing to do is probably to copy the remaining protected objects to new media, followed by recycling the source media (dunno if 6.1 already had the copy option to recycle source object protection, which is quite handy for this task).
BTW, getting 6.1 to 8.1 (or rather 8.11, which released a fortnight ago) is quite involved. Maybe you should go to 7.03 first, which is manageable and a really stable intermediate. You have to go in steps anyway, 6.1 directly to 8.1 is not supported.
HTH,
Andre.