There is the rationale for QE3: now no one other than the Fed will buy our paper.
They could simply print new numbers on old bills to save money, you know, "Reduce, reuse, recycle."
Heck, they could even have a look-up table for the current worth of the denomination affiliated with the number on the bill. Or, how about a futures market on what denomination will be assigned to an existing note?
The possibilities are endless...