What don't you understand about the OP's post??? He will still have data on the other line anyway because this is just like using another line's upgrade eligibility to get him an iPhone. If he decides to not add a line, only one of the two lines has data. If he adds a line and removes the data, he will still only have one line for data. Either way, he just wants to be left with the iPhone 4 on his number.
Thanks, but I very well understand what he's trying to do. My point is that, if this were so easy, many more people would have been doing it all long. I understand his bait-and-switch technique, however AT&T will catch on that he is removing the data plan from the iPhone (and device that "REQUIRES" a data plan), and prompt him either with leaving the data plan intact, or forcing the ETF upon him. Either way, he'll be paying nearly as much as I predicted in posts above.
Will the OP's technique work? From a technical standpoint, yes. Will AT&T take notice and let it slide? I HIGHLY doubt it, seeing as I know of others that have called AT&T to cancel and switch data plans similar to this scenario and have been denied. This is all I'm trying to point out. It is NOT "legal" from the contract standpoint, therefore AT&T maintains the right to keep the data plan intact or force the ETF upon the OP.