As a matter of fact, yes, they probably are. Or do you think it's simply a matter of drag-and-dropping cars between FM4 an FH folders?
I don't have intimate knowledge of how the resources are set up on that level, but you needn't think too hard to come up with at least several ways in which cars should be updated for the new engine. First of all, they need working headlights. Then the shaders have to be updated and tested for lighting conditions that weren't available in previous Forza games (dynamic day-night cycle, direct exposure to headlights at night). Then each car has to be tuned to the new physics model and tested on each of the 65 different types of surfaces (+ more coming in the expansion). Perhaps they need to be tested for off-road sections where the physics model is going to be exposed to more stress than on the usual FM racing tarmac, to make sure there's no clipping between wheels and other parts of geometry or some other nasty surprises. Finally, car sounds need to be tested and tuned for the new sound engine FH apparently uses. And there's probably far more work to be done than just that.