Disney has the kiddy market, and it has the tween/teen GIRL market. What they don't have is the tween/teen BOY market. It has been a major hole for them.
Now they do.
What would be the point of buying Marvel and changing it? They want access to the characters and that's it. The more recent Hulk movie and Iron-man, plus the hype around Iron-man 2, Captain America, Thor and the Avengers movie are the root of this - not the comics themselves. In theory we could see more books and a wider variety of characters kicking back into play. It secures Marvel (who weren't hurting, but still, they have had some very down-times) and boosts the comic business as a whole.
Disney linked to Pixar, then tried to do what Pixar does (and failed). They bought Pixar and let Pixar do what Pixar does. Same will happen here. DIsney want Marvel CONTENT. They want what Marvel Zombies like because they Disney makes money off Marvel Zombies.