That is and was my exact situation...I had two payments to go...What happens is you can go through the IUP trade in now, and when you check out, they will append the extra two months payments on to your total bill...I think it gets listed as "early upgrade payment". Once you do that, it works like any other trade-in. They send you the new phone and trade in kit. Get the new phone, make sure it works and all...Send your old phone back to Apple...You can also wait the extra two months and make your usual payments, then go through the process...
Thanks for the info but for me its not giving me the trade in options for my X. it shows my wife's X(only one payment is left for it) which i dont want to trade in. i am doing this on Apple Store app.