You're wasting time on something that can be rectified in one simple phone call.
You have to do is call AT&T and tell them that at the time you were issued the credit you had asked about your upgrade eligibility status and this is the timeframe you were given and at no point in time during your verbal agreement with their agent was it discussed that the upgrade eligibilty timeframe would be amended.
It's funny how customers feel cheated and bad about some automated system changing things on them when in reality - all one has to do is get a regular southern agent during peak hours to correct the issue... oh and believe me they would correct it, you just have to know how to talk to them - explain that you never agreed to any changes on your account and the courtesy credit was given as a one time credit for all the troubles you went through in dealing with AT&T's network and customer service.
You will definitely get your upgrade date back because they want you to stay and not leave... but then again I scratch my head thinking why does this guy care about his upgrade eligibilty when he's paying so much money for service and getting ripped off every month....when you can just switch to one of AT&T's sister companies and get a lot more for much less! If AT&T is poor in you area then go for a MVNO that offers another carrier's service perhaps.