Had a "pay as you go" contract since 1997 (not with T) - they told everyone that you need a new SIM for a network upgrade which required deactivating the original SIM. New SIM didn't work in normal (Nokia 1110) phones. Then they sent SMS saying that they weren't going to honour the original PAYG phone contracts.
Deviant has a good talk about this kind of thing (including some other situations to think about) in "A Talk About Risk & Preparedness" - https://m.youtube.com/watch?v=6ihrGNGesfI
Assault shed.