Just like the headphone, you will also need to buy a new case/protector for your iPhone 7. Although the iPhone 6s and iPhone 7, including Plus versions, have the same exact dimensions an iPhone 6s case / protector will not fit correctly on an iPhone 7. Because both phones have the exact dimensions, you will easily be able to put an iPhone 6s case on an iPhone 7. The problem is that you will not be able to take pictures because of the cover of an iPhone 6s will cover the camera lens of an iPhone 7 due to placement and size of the camera on an iPhone 7.
As noted by MacWorld’s Susie Ochs, one of the main reasons an iPhone 6s case will have problem fitting on an iPhone 7 is because the camera lens on the iPhone 7 is way bigger than the one on the iPhone 6s and more positioned towards the edge of the phone. Another problem is that the TrueTone flash on the iPhone 7 is bigger too. The new two-camera system on the iPhone 7 Plus is also an issue which will definitely prevent you from using an iPhone 6s Plus case on your new iPhone 7.