Questions about prepaid and 2-year phones?!

The Samsung t369 is a prepaid phone for TMo that sells for $80.
Would this phone work for a SIM card that is not prepaid (family plan)?

I am not happy with TMo's current line of phones. My Gravity T was free under a new 2-year contract, but I no longer want it. Costco allows 90 days to make up your mind on the phone. If I were to say I don't want the phone or any phone at all, will the new 2-year be voided? If no, do you think I'd be allowed to wait until a phone that I do want is available and then get that for the 2-year price?

The reason I want the t369 is because it's a decent phone at a cheap price and if I can get it to work for my current SIM card, then I was hoping I could wait until a more full-featured phone comes out and then switch to that. Anyone know?