It wasnt all that bright to renew your contract. But, i just switched from tmobile to verizon an bough the iphone 4 for $ 200 ! I love it!! Verizon & at&t prices r roughly the same so i suggest you go with the better service for $5 more (verizon)!
Since you are under contract you will need to pay Sprint to buy out your contract. Look in paperwork it should state how much it will cost. Then just go to ATT and they will do the rest. You can keep your same number.