Jesse McCartney, who was considered for Prince Zuko, had scheduling conflicts and couldn't take the role.
His replacement is British actor Dev Patel.
Which is an interesting casting choice because Zuko is a member of the Fire Nation, who are seen as the antagonists of the Avatar world.
Race in Avatar
As much as we'd like to believe that they're actually casting 'authentic Asians'--as opposed to fake Asian?--in the movie, this recent casting call has only led us to realize one thing: Paramount Pictures intends to cast according to the races of the lead roles. Therefore:
For more on the subject, see The Best Indian Movies.