Remember the bad old days of the iPod? Apple would release a new model with a bunch of new features, and the old model – while capable of running the new software – would be left out in the cold. It was obviously a cynical scheme to make you upgrade your perfectly good music player early, but it worked.
So it is with cameras. Firmware upgrade are almost always limited to small bug fixes. The latest 7D update, though, is huge, and almost gives owners a brand new camera.