Oh well, we already switched off of T-Mobile. They refused to prorate our final month's bill, too, telling us that paying the full bill is the exit penalty (and is in our contract). I have a hard time believing that, but I really don't know what to do that isn't more trouble than its worth.
If I remember the contract correctly, the purchase being blocked was the best case scenario for T-Mobile. There were clauses that would strengthen them through ATT network access in case this happened.