Hey there, great question about JOD! As I understand it, the Apple program is completely independent of any carrier financing program. Because of this, you are still obligated to complete your Jump on Demand agreement with T-Mobile.
This could be by completing (or paying off) your 18 month agreement and returning the phone to T-Mobile or by obtaining the "End Lease and Purchase" price from T-Mobile and paying that full amount. This method allows you to keep the phone to do with as you wish.
Manage your T-Mobile account