The iPhone 7 and 7 Plus have the exact same dimensions as the iPhone 6s and 6s Plus, down to the tenth of a millimeter according to Apple. So you won’t need a new case if you’re upgrading, right? Wrong.
iPhone 6s cases probably won’t fit the iPhone 7 because of the camera bulge on the back of the phone. The camera lens on the iPhone 7 is significantly bigger, and also located a little closer to the edge of the phone. The TrueTone flash on the iPhone 7 is bigger too.