Promo credits generally have a clause that says you need to be a customer in order to get the credit. Especially if it's designated as a monthly thing.
From the Support Document:Lifetime Coverage Guarantee:
You can also keep your JUMP! On Demand lease payments, even if you switch to another carrier.
Note: If there was any promotional pricing attached with your device and you decide to leave T-Mobile, that promotional pricing stops, which will result in an increase in the amount of your monthly payments.
The general rule seems to hold.
Best of luck in your new home!