One wonders if the replacements are moving on to facilitate the return of the originals.
I tend to look at it this way; NBC renewed the show in hopes that it will grow it's demographic the way Seinfeld and some other shows have. My guess is that they were well aware of what the long time fans were saying about this past season, which was the same thing that was being written right here.
NBC must realize that to maintain the show's uniqueness, they had to bring Harmon back. That is one of the show's main hooks. The (former) current crew were just not up it. Harmon presents the best path to growing that loyal fanbase.
I'll bet TPTB simply told the staff you can either bow out gracefully or be fired. If they fired Harmon, you know they wouldn't hesitate to fire the new guys. You know that if the ratings and digital word of mouth had improved enough (neither did), NBC wouldn't let those guys go and with only one season and a renewal, why would those guys leave voluntarily?
They got the boot.