I think the thing I understand the least about this change is, why wouldn't they remove Ben Kenobi and put in Ewan McGregor's Obi-Wan? I mean, if you're going for their "young person" actor.
Probably just has to do with him returning to the Light side and that was his last form as a Jedi. Obi Wan never went bad. Or something, I don't know. That man is on crack.