I really think the "reason" is Lucas wanted an excuse to put Christensen in the movie, since he wants that to be the way the character looks.
Anakin is supposed to be redeemed before he dies, it would seem like that would be enough to have him look the way he alsways did.
I think it makes sense thematically to do make that change, based on the whole "freezing spirit" idea, but I still have big issues with changing the actual content of the movie. Again maybe if they had made the original version available as well I wouldn't feel that way but when I get to the end and see Hayden smiling down on me I kindof feel cheated.
I hope all the questions are answered on Thursday as well! Sorry to be a downer for a minute there.
I stand corrected, you are as vapid as they say.