Boy Meets World
Little sister Morgan is the hilarious and very loud-mouthed sibling of Cory on Boy Meets World, the hit teen comedy of the 1990s. However, she looks entirely different going between seasons two and three. This is because actress Lily Nicksay portrayed Morgan during the first two seasons, but actress Lindsay Ridgeway took over the role after her, as the character needed to grow up faster.
The O.C.
It might seem impossible now, but once upon a time, actress Shailene Woodley was recast from her role as Kaitlin Cooper on The O.C. She was replaced by Willa Holland. Shailene just wasn't a good fit, and "the version of Kaitlin Cooper that we were going to bring back to the show was a little bit older than Shailene would’ve been able to achieve on her own."