Again, only requirement here is that you keep the line active and keep up the device payments for 2 years. If you want to upgrade next year, you can do so (e.g. sell iPhone 7 and buy 2017 iPhone model for full price) while continuing to pay for device installment to continue receiving the credits. Pretty much if you're smart about how you upgrade next year and don't need to switch carriers, you could essentially be getting iPhone 7 for just the tax.
Exactly what I plan to do. Even if I don't, no way you can get $650 for an iPhone 6 Plus that's 2 years old. Going for eBay around $400 (give or take). Even if I trade next year and lose the rest of the credits, still get over $300 in credits. I like the deal myself.