One wonders if the replacements are moving on to facilitate the return of the originals. Would be nice if they were that gracious, but a job's still a job...
Everything I've heard about the behind the scenes of Community suggests that it's a horrible place to work as a writer. During the first three seasons this was put down to Harmon's managerial style (which is something he himself admits he wasn't good at) but the atmosphere didn't seem to improve under Guarascio and Port. Given the stress and long hours they would have endured only to end up with a lacklustre season that disappointed fans, I can see why Guarascio and Port would choose not to return for season 5. I believe some other writers also left after season 4 for similar reasons.
With the new showrunners gone, Sony had a dilemma in replacing them as most of the senior producers on Community left after season 3. They'd need to hire someone in from outside again, which hadn't worked so well last time they tried it, and it would probably piss off fans even more if they hadn't at least tried to bring Harmon back.