I wanted to mention my experiences with Apple's Upgrade Program vs. what I went through for years with Verizon, who started offering me yearly upgrades with the same general kind of financing long before Apple did:
Verizon:
The first year I ever got to skip from one year straight to the next was when I went from the iPhone 6 to the 6S on an "early upgrade" offer from Verizon. That year everything went smoothly. The next year my wife joined the fun, and by the time we had received and activated our 7's, we found out somewhere between USPS and the Verizon return hub, our old phones were lost. We were charged full remaining value for both and it wasn't sorted out for months. Our service was shut off during vacation because they claimed we owed them money for phones we sent them and had receipts and tracking info for. Long story short, Verizon did eventually do the right thing and just wipe out the balance but that was after a lot of stress and arguing on the phone with both them and USPS. Every subsequent year there always seemed to be some kind of problem with activation, confusing billing, mixups, delays in receiving old equipment, etc. Verizon just never seems equipped for the influx of used devices and it always bites US in the end for some reason. Their support and service seems to have improved over the years. Maybe this program has too. I'm not optimistic.
Apple:
After new iPhones are announced, I get an email letting me know I'm eligible to upgrade if I want. A day (or less) later, I'm asked if I want to set up the trade in advance so I can just hit the buy button on launch day. On launch day, I hit the link from when I set up the trade in, I hit buy, I'm done. New phone shows up, old phone goes back to Apple, I'm a happy camper, and I know my old phone doesn't end up in a landfill and will get used by someone else. I have never had a billing problem, confusion on pricing, activation issues, or anything else go wrong yet. The ONLY annoying thing is that I'm forced to have AppleCare+. I've broken one iPhone in my entire life and really don't feel I need it.