Hello Mike,
Trick question. If you want to do it as you describe, you need tooling. Check with your Veritas SE. Veritas bought Globanet, they have a tool called Classify. I am sure there are more tools, and possibly you can 'hack' SQL to do it, not sure.
Long route, create a new Vault Store, create the 5 year retention category, use Move Archive to put those abandoned archives into a new archive (with 5 year retention).
bit more:
If your storage is on WORM for your user archives, you can change the forever retention to 5 years (or whatever is required), but WORM takes precedence. If that also has 'keep data forever', then you cannot delete items from it.
What I would do is to define what the longest retention needs to be (check with your legal department, and the business units). CHANGE the forever retention to reflect that longest period. Changing the retention category is NOT reactive. Items archived with that category when it was forever, remain 'forever'. Only newly archived items will get the correct retention.
You might want to explain what you want to achieve (expire items from those old archives? Remove them? use more retention categories?) to allow the colleagues in the forum to properly answer.