number portability to "old"phone

I understand (more-or-less) that if I buy a new phone on a new network I
can transfer the number on my old phone by obtaining a PAC code from the
old network. Is there a time limit during which this process has to be
performed? In other words, how new does the new phone have to be? Can
I use this mechanism to transfer a number to a phone I've used for
several months?