So far as I know, there's no way to switch between services. My brother asked about that at one point and was told it'd be easier just to sell the phone and get a new one. And TracFone in particular uses their own technology, I belive.
But you might Google newsgroups or sumpin and see what people say.
So a Tracfone Nokia works different from a Verizon Nokia, and likewise the Motorolas?
(grump)
OK.