I applied to CX back in 2000, got called for a phase 1 interview but turned it down as I had already accepted a job with AA. When I got f'd from AA after 9/11, I re-applied to CX and my original PRN was still mine. Re-applied again a few months ago and used the same PRN on the paperwork. Did not hear anything from CX saying a new one had been assigned, so I'm assuming it's still mine.
So, long answer to a short question, but I'd use the original PRN. If nothing else, maybe a well-aged PRN shows interest in CX over time...?
Mink