Traded in iPhone 8 Plus before paying it off

I have a billing question. I recently traded in my iPhone 8 Plus to get the new iPhone XS, however I still have payments remaining on the iPhone that I traded in, as I bought the iPhone 8 Plus through AT&T during a BOGO promotion. The iPhone XS I purchased is unlocked and I bought it from the Apple Store directly so I do not have any more payments to make for it. I am wondering if doing this will affect my bill and how much I will have to pay or will I be able to simply continue making the same incremental payments that I have been making for the iPhone 8 Plus even though I have traded it in.