No where, but that's not the point. The question is whether or not this change is a materially adverse change to the contract, thereby giving the OP an out to the contract without incurring an ETF.
I'm not sure if the term "materially adverse" has been thrown around on the message boards, and hence the OP's familiarity, but it is a legal term of art and there are many cases on it.
I would say that it is materially adverse, but you are going to have a hard time explaining this to a store manager let alone a sales rep. You would have to go all the way to corporate, to someone who understands this term/legal question, and even then you would have to convince them it is MA.
Could the OP point to the MAC in the contract?
I don't recall AT&T's upgrade policy ever being printed or used as a sales incentive. More often than not, it was used and considered a bonus, an extra, rather than something guaranteed.
That said, even if the OP were able to prove that it was materially adverse, the change occurred months ago. If they wanted to get out of the contract with no ETF, that would have been the time to do so. By waiting and not doing anything within a timely manner, they (for all intents) agreed to the new terms.
At the end of the day, regardless of what the sales people say and ignoring what AT&T may (or may not) have done historically, all that matters is what's printed on the contract the OP signed... the one that says 2 years, no early upgrades.