Can you just walk into a Cell phone company store with any phone and make it a prepaid phone?

I have an old smartphone (Samsung Blackjack 2) that I used to have on a 4 year contract. The contract is over - it does nothing now. I would like to use it as a prepaid phone... is this possible? Can I just go into a Rogers outlet and make it happen? I'd really rather not buy a new phone... I love this phone.