I was actually speaking more generally, responding to a post that wasn't strictly about CBS-FM. But you're right. From what I hear CBS-FM has been billing great. With the 60's music. ;-)
But if they want to keep the 25-54 ratings at the current level, and to retain billings, they have to regularly adjust the music to keep their audience from becoming older as the music ages. That means occasionally purging the older stuff that the younger side of the station's target do not particularly like.