Canon has already done it with the 60D. The 60D has less feature than the 50D and lose the metal body at the same time. Logically, 60D is supposed to have more feature and a step up from 50D. 60D basically has changed the tradition and linage of the XXD. Now 60D has been nicknamed "Super Rebel".
"Less feature" on the 60D is very nearly a matter of interpretation. It doesn't have microadjust (important to be sure and rather inexcusable to lose on a camera of this rank) and, almost unimportantly, the magnesium has been replaced by polycarbonate, glass fiber, and aluminum. (Moving from aluminum to carbon fiber in tripods is one thing, but in camera bodies going to aluminum seems minimally important, as aluminum is still plenty good as a heatsink.) It also loses one frame per second shooting speed, going from 6.3 FPS to 5.3, and the control wheel. That is really about it on the negative side.
The 60D "has more feature" than the 50D in every other meaningful way. Each little point, taken individually, is small, but taken together they lead to a camera that really is a replacement for the 50D, even if Canon has been careful not to position it that way (that's what the 7D is for). When the 60D gets its own replacement, the new camera will probably be an unambiguous upgrade from the 50D in all areas but a few specifics to annoy the most finicky of purists.