Paying up a contract and converting to a prepay plan. Possible?

To terminate a contract with a cell phone carrier, you have to pay a termination fee. The amount varies from carrier to carrier. Now, if you have a phone that was purchased through the payment installment program that they offer, you'll have to pay that up in full, along with the termination fee. As far as switching, I'm not sure. I'm guessing that you'll have to get a whole new phone number when switching to the prepay service.