I know Sarah Chalke first played Becky cuz Lecy Goranson went to college, but why did Sarah Chalke play becky in some of season 8 and all of season 9?

In season eight Sarah CHalke played becky in 3 episodes because Lecy was at school.Season eight was supposed to be the last so Lecy had other commitments when she learnt about the nineth season so Sarah was in season nine.

Did anyone see the movie Dying to Belong With Sarah CHalke?