I'd be curious to see old music logs for when exactly a station makes these changes. A lot of times I notice theyre accompanied by either a slogan change or some kind of change like "now with 10 songs in a row" soas its not as noticeable.
When I worked at a station that went from Mainstream CHR to Rhythmic CHR about 20 years ago, here's how we did it. As David mentions, there's no right or wrong way to do it, but here's what we did:
1) Adjusted our music mix from 6:00 PM to 6:00 AM immediately. Went from a standard CHR rotation to 3 urbans and one pop half the day.
2) Softer pop hits, like Sheryl Crow "Soak Up the Sun" and "The First Cut is the Deepest" were immediately dayparted to 10:00 AM to 4:00 PM Monday thru Friday. Softer pop hits more than 18 months old were spiked.
3) Changed our nighttime programs to edgier names. "Make It or Break It" became "Q It or Screw It." "Demand Radio" became "The Booty Call," where, instead of demanding your favorite songs, you had to tell a relationship, love or sex story to get your song on-the-air.
4) A few more rhythmic songs were added, mostly at the expense of pop gold tracks, in morning drive and between 4:00 and 6:00. Those dayparts became more rhythmic over time while the midday show began sounding more and more like the morning show and afternoon drivetime.
What we didn't change:
1) Positioners. We were always "THEEEEE Hit Music Channel" and "THEEEEEE 20-in-a-row Hit Music Channel."
2) Personalities. Well, not immediately, anyway. Shortly after the switch, our morning show, minus the female sidekick, moved to another market. Our PD/afternoon personality and night guy/MD were the morning show when the station was on a weaker signal. They went back to mornings. The morning sidekick picked up a few hours of the midday shift. Afternoons went to the midday guy, and we had hired a host of an urban program on the community noncommercial radio station for part-time/weekend work and promoted him to 6:00 PM to midnight.
3) Imaging. Well, okay, we made a few changes, but they were subtle. Softer and slower jingles were dayparted to 10:00 AM to 4:00 PM before being phased out altogether, and we added a handful more of what we referred to as "Quick Hits," which were station liners between songs.