All was in a change. In 1991, most characters had oval shaped faces and were much shorter and thinner. In 1993, the design clumped and it was wider. They also became much more different, as they seemed to have less caring and sensitivity. In 1997, they changed even further. The characters were also less mean, and they got to it by 2001. There they expererienced more changes.
In 2006, the characters looked regular but continued to evolve in 2011. They seem to be more different, and Chad's appearance went further than anyone else. He is now a major character, and he is also playing a much larger role, a little more than in the books.