Well, if they just go through the maps and update the graphics then they could probably do it. Mod people made Morrowind in Oblivion... (Well enough that you could enter the province, not the whole thing)
Exactly. They couldn't do the whole thing. It is just not worth it to do so. Also the physics engines are totally different, so you just can't put Morrowind's physics, with Oblviions physics, as was asked numerous times before. You were not the first, and you will not be the last to ask this.
It is not an easy thing to do, or it WOULD have been done now by modders. Since the closest thing we have is Morblivion or what ever it's called, you can see it takes alot more work than previously thought.