Your phone number is tied to your SIM, not your handset. If your upgraded phone takes the same size SIM, just pop your old one into the new handset. There's also a chance your old SIM could be cut down to a smaller size and reused.
Otherwise, you will have to purchase an appropriately-sized SIM and get *611 to switch your number over to the new one.
In either case, your phone number, account balances and billing details will remain the same.