I would reach out to Tmobile through social media and ask for help.
You do need to pay off the phones once canceled and the one line that's not being used can be canceled with an eip and you can still make monthly payments.
There are definitely a lot of questions here. First of all, there is no early termination fees (ETF) as that is the term used when canceling a service contract and we do not have those. There is however the cost of your phones if you opted to pay them off in installments. If you cancel all your lines, we have no way to continue the monthly installment plan you agreed to and the remaining balance of the devices becomes due on that final bill.
As for changing your plan, you can always change your plan. What plan are you trying to change to? More importantly, what is the debacle that you are wanting to cancel your service over?
If you would like a device payment plan that is independent of your cellular service provider. Amazon, Best Buy, B&H Photo Video, and many other retailers and e-tailers have plenty of payment options and devices to choose from.