So what will happen now is when you request to cancel, they wills et it up for the end of your billing cycle. You can request to have it canceled today. Then 5 days after your billing cycle ends, you will receive a final bill that will contain your final balance, including anything owed from the Tab & usage.
Usually you can't come out of the contract until it expire.. you have to put forward a request to your SP and terminating contract will end up in contract termination fee.. Instead you can simply change carrier by unlocking your phone from any vendors online like http://www.prounlocking.com