You are not in contract. There is no case. AT&T can crank the prices for everyone up 1000% tomorrow who is not in contract, and there's no legal remedy for that. AT&T as a private, for profit company is not beholden to anyone but their voting shareholders as long as they operate legally. A rate plan increase for out of contract customer with several months of clear warnings is quite legal, however upset about it you are.
My statements and opinions do not reflect those of my employer, AT&T Mobility. This is disclosed per FTC regulation.