Post below is MY experiances only and is not suggesting any behavior by persons or entities....
I see so many posts all the time where people are saying they didn't get what they thought when upgrading or starting service with Tmobile. It's actually very simple to avoid any confusion or bad feelings by doing a few simple things.......
First off i ALWAYS deal directly with TMO corporate for anything. whether it be a corporate store or the official tmo ile website. If I see it on The website then I call in and get the deal. Basic add ons are fine online but major changes or phone purchases I do over the phone. Tmobile records calls and anything said can be tracked if there is a dispute in the near future. Don't worry... They are very fair and if they listen to the recording and when correct ..... They honor what you were told.... Within reason of course because of course if some crazy operator flipped out and promised you a car you should have known better.
Buying in a local store you should Never be afraid to make notations of promises and plan features in your contract margins. If your promised $20 data then write it in a margin and have salesperson initial. This applies to ANY thing promised that isn't in black and white in the contract. You can also write in things like " all previous contracted pricing and features continue with no changes" just make sure to make a quick precise notation about anything questionable or important and MAKE SURE to have it initialed by both parties.
If a salesperson say you can't do this then WALK AWAY. There is no contract police that comes and arrests you for making notations

You will find that anyplace playing it straight with you will not mind you doing this. They might roll there eyes a bit but will initial the notes if they promised you something.
All this doesn't guarantee you anything in the way of a deal that honestly can't be done.... But does give you a leg up on any crazy charges made from misunderstandings with an option to go back to square one and make a new informed decision what you want to do.