I did the upgrade this morning ... thanks OP!
Here's my experience.
Called 611, then 0, then 3. Connected with a pleasant CS/Retention rep. Gave her my info, told her I've been seeing ads from Sprint that are offering 20 GB.
She looked at my account and said that she was going to look at the various plans available. Then she asked me what was appealing about the Sprint deal, to which I said that they are charging the same money for twice the data.
At this point, she says that there's a promotion that would double my data to 20GB without increasing the price. After some more small talk, I agreed to the change.
She mentioned, several times, that this data promotion would be in effect permanently unless I changed the plan in the future, in which case, the price would be dependent on the pricing/promotion available then. She also mentioned a few times that this promotion was good until 10/31.
Here's where it gets strange. I check "myAT&T" app and indeed my data allotment has been updated to 20GB. But I get an email from the rep that I talked to saying that they will be cancelling my line as discussed and that I have 59 days to re-instate the line if I want to come back.
After seeing that email, I check to see if my phone still works ... it does. I call the number on the email and talk to a rep to figure out why I got that email. She confirmed that there is no cancellation order and after a little investigation, she says that since I called directly to the "Cancellation Dept", that all phone calls will send out the cancellation emial automatically.
So I'm good with the extra data at the same price.
Also, I was able to confirm with the first rep that my 25% FAN discount would still be intact.
ft