How do trade-in upgrades and installment plans work?

I am interested in trading in my iphone 6s plus to a iphone 7plus. My current 6s plus is on an instalment plan (on att next every year,) where i can upgrade at 50% with a trade in. When i upgrade to a new phone do i have to pay off the remaining of the instalment plan for the 6s plus? And when i upgrade to the new iphone 7 can i add that phone to an instalment plan instead of paying it all off at once?