If your strictly looking to save some $ and are completely happy with the 15 (no major issues etc at this point)....and assuming from the sound of your post that you still have a ‘note’ on the 15...you could always talk to your bank about refinancing the 15 to a lower monthly payment (wouldn’t save you any $ in the long run due to additional interest from a refi but could save you some decent $ in terms of a monthly budget).. just a thought.