They did this to me years ago and forced me off my unlimited data plan and switched me to a limited data plan for more money. When I complained because the letter I was sent said my plan would have the same benefits but could cost me a dollar more. They gave me back unlimited data but the other device on my plan still lost the unlimited data.
Right now I have 8 lines on my family plan and pay $160 so I will NOT be switching, fuck those guys.
That might be more difficult because prepaid users are quite a bit more cost sensitive and since most prepaid is bring your own device they are not locked in to the T-Mobile network like postpaid users are.