[otrs] Changing ticket number generator

Hey, we are using OTRS within our team for some time now and are quite happy with it. One annoyance is the Ticket Number format (DateChecksum) we chose: As we frequently get calls about the tkts as well, it is always problematic getting a 16-digit-number through a phone line. Thus, I am considering switching to a new format or even implement our own. My concern is about the pre-existant tickets: Some of our tickets have pretty long lifecycles, so it would be mandatory that OTRS is able to assign incoming communication to the old ticket number, while as well recognizing the new format for follow-ups on new tickets. Has anybody ever done this or does anybody knows the pitfalls to face here? Brgds Daniel