You should call T-mobile and talk with them. Most BB data plans with the major carriers require a 2 year contract. T-Mo is sometimes a bit different, but I do not know what has transpired since the recently announced acquisition by AT&T. I doubt nothing has changed yet, but you should call and see.

You will also need to check your contract with Wal-Mart to see if you can cancel or if you can transfer to T-Mo, since they use T-Mo's network.