In Season 1, Carly seemed very preppy (Briarwood), yet in Season 5-6, she was more... for lack of a better word, non-preppy.
In Season 1, Carly immediately helped Freddie up when Sam bonked him on the head with a plastic/steel pencil. In Season 6, Gibby knocked Freddie unconscious and Sam got up to check on him while Carly did not.
In Season 1, Carly and Sam would fight over boys (and Carly easily had more love interests than Sam did). By Seasons 5-6, it was becoming a running gag that Carly couldn't maintain a serious relationship.