6. Lily - Modern Family
Modern Family is a show largely populated by young people, with each of the three families featured having at least one kid. Child actors are the most vulnerable to being recast, either for their acting abilities not progressing as well as they should or their looks having changed into something that doesn't quite fit anymore. So the fact that Modern Family only ended up needing to replace the twins playing Lily, the adopted Vietnamese daughter of Cam and Mitchell, was actually pretty lucky. There was nothing wrong with the two little girls, they just didn't seem to enjoy the day to day rigors of on set life (or more likely, their parents didn't), so the decision was made to bring in a new Lily, Aubrey Anderson-Emmons. She quickly grew into the role, allowing the writers to develop an acerbic wit for the young character. These days, with the writing staff not really knowing what to do with a teenage Manny and Luke, she's frequently one of the funniest parts of the show.