If I remember correctly, I had just gotten my iPhone 6 plus when they came out and then went to Disney that next week and they didn't have anything for the 6 plus so I expect the same thing for the first few weeks (at least) with the iPhone 7. My iPhone 7 doesn't ship until November so I'm going to use my trip this weekend to scope out the case options and then maybe get one on my next trip.
Disneystore.com doesn't have any yet so my guess is the parks won't either this early. So it's just another excuse to go again!