Will my AT&T service be terminated if I switch phones?

I have the Blackjack 2, on AT&T pay-as-you-go service. But now I want the Samsung Impression, haha. If I switch take my old sim card (pay as you go sim) and put it in the Impression, will my account and money left be carried over? Or do I have to start a new pay as you go account and get a new number?