Samsung SGH-T309 / T319

I recently saw a prepaid version of the t319 at wal-mart and thought to myself if it would be possible to simply insert my current t-mobile simcard and put it in the t319 without having to pay a heavy pricetag or resign a contract. However i've heard that carriers wont let u simply switch your current simcard into a prepaid cell phone, is this true because if not this camera phone is at a really good price for a prepaid phone.