We come across this thing a lot at our Museum as well. Regardless of what you decide to do, the most important thing is to make sure that you document your decisions thoroughly in the record. Past Perfect makes this pretty easy. There is a Old# data field, as well as field for notes.
I think that it would be a nightmare for you, and confusing for future employees, if you try to make up new “71” numbers for your items. I recommend renumbering them with 2013 year numbers, and lot numbers that reflect which items came into the museum together. Each item can then be given its own unit number.
Just be very clear in the record about what the original numbers were, when the items came to the museum, and why the numbers were changed.