Not going to happen on AT&T. If you downgraded the data on an upgrade, you're stuck with it. Sprint, Verizon are the only ones offering the unlimited. Verizon is rumored to go the tired data pricing in Q4 or Q1.
If you signed for the data on a new purchase or line there is no unlimited anymore with AT&T.
Also just an FYI, there really is no unlimited data. They cap it at 5GB. Anything more then the 5GB and they trottle you back. Sprint will text you a nasty gram. I just jumped the AT&T ship. They made upgrading and keeping the unlimited data a real hassle. They tried every trick in the book to trip me up on the upgrade. 50.00 more, 100.00 more, 100.00 rebate, then the crowning blow they tried to tell me I had to use the 1400 minute plan to get the phone. The Verizon store is right next door and I ported the number. And walked back over to the AT&T store and rubbed the entire thing in his face. At that point they wanted to deal. You know what I said. You know dam well what you were doing to screw me, now you have lost me as a customer. Bye-Bye AT&T!! I took 5 lines over to verizon, plus a data card. I got the last laugh.