Now is you know which one (I'm on T-Mobile, BTW), you should be able to switch. I'm in the process of switching. T-Mobile has better pricing AND no equipment (modem) charge. No data cap, no contract, no up front fee. If you are a heavy =user you could see slower speeds after 50GB per month.
You might need to talk to a 'person' if site shows service not available (you know better, you already have/had service).