Unlike the update from the iPhone 6 to the iPhone 6s, where the only changes seem to be internal, an increase in the number merits a physical design change. This was true from the 4 to the 5 and the 5 to the 6. The new iPhone 7 may look similar to the iPhone 6, but Apple has changed an important part of the phone that makes all iPhone 6 cases incompatible with it.
If it wasn’t obvious already, the camera hump has significantly been changed in size, now much wider with a smooth bezel around it, compared to the hard edges on the 6 series.
This will most likely make any case you have incompatible. Other changes such as a lack of a headphone jack and the addition of a speaker won’t make much of a difference, but the physical limitation is even more evident in the iPhone 7 Plus with the new dual camera system.
iPhone 7 Size:
iPhone 6s Size: