Phone numbers aren't hard-wired into a SIM, so Cingular should be able to assign your old number to the new SIM. If you maintain/extend your old contract they should be willing to accomodate, I guess. You probably won't be able to port your old CIngular number to a new Cingular contract.

If you want to port your old number to a contract with another carrier you must do this before cancelling your Cingular contract. You'll also be hit with an Early Termination Fee from Cingular if you cancel within your contract term. Otrherwise there's nothing that prevents this.