The parts of Lewis and Wilbur had to be recast when the original actors' voices broke.
After reading the screenplay, director Stephen J. Anderson actively lobbied to direct this movie. As a child of adoption himself, Anderson personally experienced many of the emotions (questions about belonging, being wanted, etc) that Lewis expresses in the film. [source: director's DVD commentary]
The character of Lewis was voiced by both Daniel Hansen and Jordan Fry. Daniel Hansen voiced Lewis at the beginning of the film's production, and when Lewis needed things changed, they had Jordan Fry re-dub some segments. It is noticeable in some parts of the movie.
The science fair that Wilbur attends is held at Joyce Williams Elementary school. This is a nod to the author of the original story, William Joyce.