The T-Mobile web site validates what @BasicGreatGuy posted. You can activate a new line OR trade in an eligible iPhone. They also state the trade in price depending on what you have to trade.
There are some other things though to consider: The SE2 is not in stock yet and any price deduction offer will be given to you via bill credit. I'm not sure if T-Mobile will allow you to pay up front for the SE2 (after any offer) or if you must accept bill credit.
I'll find out tomorrow when I drop in on them and ask.
So if you cancel before 24 months then owe the full price $399 instead of $199.
I am considering the new SE but would want an option to upgrade next year, so not sure if it's considered cancellation or upgrade. I wouldn't mind paying $199 for SE instead of monthly installments with no contract.
I am holding until SE reviews come out and any new s on SE plus which might have a dual camera as 8 plus.