The Cool Thatguy was speaking about Batgirl 2009, where Cass left the Batgirl mantle to Stephanie without an apparent reason (later, it will be said that Bruce told her to do this in case of his death).
Cassandra becoming evil happened in Robin OYL (2006), but it's retconned to be caused by Deathstroke's magical drugs later. In 2009, when Cassandra left the Batgirl mantle to Stephanie, she was good again.