Depends on the character. Like you said in soaps-more so than any other series. They change either because the actor decided to leave(many), died(Edward Quartermaine comes to mind), or taking the character in a different direction/aging the character. Unless the actor just doesn't do well, viewers tend to accept it....even if it takes awhile. Primetime for whatever reasons, viewers are a little pickier. Even when it's not an actor replacing another in the same character. Like when a character/actor leave, they bring in someone to be a new character-viewers always still compare them.