All was in a change. In 1991, most characters had oval shaped faces and were much shorter and thinner, as well as big noses. 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 somewhat different, but Chad's appearance was the most drastic. He is now a major character, and he is also playing a much larger role, a little more than in the books.