Sadly you are SOL. You will just have couple of choices. Since you said you are on AT&T Next, you have the option to pay to upgrade, so you will have to pay the 12 or the 18 month worth of the installments and then you would have to trade in your 6 and then get the 6 plus. Other option is to pay the remaining balance of the NEXT and then sell the phone and get the 6 Plus.