I'd have to agree with everyone who said that the characters on Buffy and Angel all seemed to change a great deal. So many characters made massive changes over the years but fans went along with it because the characters matured and most faced huge obstacles.

Agreed. The character arcs in Buffy/Angel always seemed authentic and well-thought out. Never cheap. In some shows, it seems as thought characters change simply because some executive sat in on a focus group and made a snap decision like, "make hot daughter a little sluttier."

Pacey was a pretty big change. From the wittily verbose slacker sidekick to a sleazy goateed boiler room broker to the good guy who ultimately got the girl. While we're on Dawson's Creek, as Van Der Beek went from the titular star to a supporting character, Katie Holmes was somehow determined to the breakout star, so the was the focal character the last few seasons. It morphed into Joey's Creek featuring Dawson.