Happy Days - Potsie started out as normal and got stupid. Fonzi started out cool and got stupider late in the series.
Boy Meets World - Topanga started out as the hippie, earth child but changed to being the studious, level headed one. Also, her relationship with Cory changed from contentious to them being married but in the later episodes the writers tried to act like they had always liked each other and that their relationship in the earlier episodes didn't exist.
Frasier - Daphne started out as spunky and funky and changed to boring after she married Niles.