I read about this on MacRumors and even on the actual AT&T community forums. You have to call the retention department and tell them that you're thinking about canceling and going to Sprint, because they offer 20GB for $100. Sometimes you will get lucky and won't have to threaten to cancel, and sometimes you may have to try a few different retention reps. You have to make sure that you're speaking with the retention department, because normal customer service won't usually do it and won't know what you're even talking about. Don't try the chat session either, because they are normal CS.
::EDIT at 8:20am:: I just called and selected the cancel service option (you press 0 and then 5), it sends you right to the retentions department. A nice lady answered right away and all I said was that I seen that Sprint is offering 20GB for $100 and I might have to take that offer if she couldn't give me any other options. At first she tried pushing the 30GB for the price of 15GB, so I said that I really don't want to pay more. That's when she said that she'll check what options she can do. She went on to say that she can offer 20GB for the 10GB price (which I didn't hear her at first) or I could go with the regular promotion that's out right now. I asked her to repeat the first option again and that's when I heard her more clearly. I jumped on it right away.
She said it wouldn't start for me till November 7th, which is fine. I was sent a text message and email about me changing my rate plan. Before she hung up, she said the promo she gave me was exclusively for me. Lol! So hopefully it does change over on the 7th (since it's still showing 10GB right now), but I did get the two confirmation messages, so it should.
::Another Edit - 10am:: I decided to call CS back to make sure I am going to get this 20GB for the same price as 10GB. she said it shows in the notes that it would change over on the 7th of November. I then asked if I could move the date up to today and she said that I would have to speak to retentions again to change it, since they were the ones that changed my plan. So I was transferred over and spoke to another nice lady. I asked if there was anyway I could have it start now, she said yes and that she'd do it for me. Great! She then said that since I haven't gone over my data for this month that I won't have any prorated charges at all. Another great thing. So she changed it and I asked if it would reflect right away if I checked my account and she said yes. We went through the polite ending talk and the call was ended.
I then went to check my account online, it's now blank above where it used to say 10GB Mobile Share plan, but it still shows that I have 7.3GB left (like it did before I changed to 20GB). On the side it says that I recently changed my plan, which I click on it, it takes me to the normal plan page, but doesn't show any plans, just shows a red box and it reads - "Looks like you have the only Mobile Share plan currently available to you. For further assistance, call 1-800-331-0500"
I asked someone at Howard Forums about it and they said that theirs says that too, but that just means if you ever want to change your plan again, you can't do it online, you have to actually call. I logged back into my account on my phone this time and it's now showing 0.03 out of 20GB. Awesome!
So when you guys call in for this, make sure you ask for them to start it for today, instead of the next billing cycle. It doesn't cost anything if you haven't gone over data yet.