According to the rumors and speculation, Apple is not expected to introduce major design changes to the iPhone 7s and 7s Plus unlike the iPhone 8. However there will be some differences, such as the use of a glass back panel that is meant to help facilitate wireless charging, and because of that it has since been rumored that the iPhone 7s could be thicker as a result.
This is according to German blog Giga Apple who cites a case maker who claims that the new iPhones will be thicker by 0.1mm than its predecessor, which by the way was also thicker than the iPhone 6 due to the introduction of its 3D Touch display. While this might not seem so obvious in your hands, it could mean that you will need to shop for new cases.
This is because cases are usually designed to be tight fits, so even the slightest changes could mean that it will no longer fit. Take it with a grain of salt for now, but hopefully Apple will have the official details for us next month.