Remember, stations do listener-based research to determine the songs to play, which not to play and even which should play more than others. With the recent change in morning shows, I am rather confident that they did considerable image and music research to insure continued appeal and to "freshen" the sound after Scott left.They change the music on CBS FM lately, more variety and not only pop/rock oriented anymore like before, even more urban/pop-ish dance tracks from the 90's with some more disco from the 80's. So it's a good mix what stations like PLJ (Hot AC era) and Hot 97 playing back in the day.
To me, that is not “changing” the music but using scheduling techniques where a portion of the library plays for a defined period of time or specific number of plays and then rests for a while as other songs enter. Both Music Master and Selector have ways to easily define and do this.FYI they change the music continually. Songs are added and dropped every week.