I ordered from Amazon and have the unlimited data plan as well. When ordering, you can pick any plan, but take the phone to an AT&T store to activate it once it arrives. Make sure they know you are grandfathered, and they will set it all up for you.
I called AT&T late last night to see if there was a way for me to purchase from Amazon selecting their data plan and then still keep my unlimited plan. Before we even got into that, the rep went to talk to his manager and got approval to sell the phone to me for $99 instead. That was awesome. The bad news is that we got disconnected towards the end of the ordering process and he never called me back, and when I called them they didn't see the order was processed. We processed it, we think, but I still have no confirmation email, no order history on my wireless account and more importantly no posted transaction on my bank account. I called two more times to speak with other reps who confirmed it "looked" ordered but they couldn't tell. Instead of placing more orders they just suggested I wait and see what happened. They were positive I would have received an order confirmation email though...which I haven't still and it is 12 hours later.
I think that if it doesnt ship by 3pm EST today, I am going to call and cancel. I will then go to the AT&T local store to see if they will price match. If not, I will check out Best Buy. If not, I will order through Amazon if you are certain that I should be able to retain my unlimited data plan if I take the phone into the store to activate it.
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Go to Best Buy and get them to price match Amazon.
I believe Best Buy is price matching online competitors this year starting Nov 1, so I will definitely go this route if I can't resolve the issue I posted above.