That sucks. I liked Ann Perkins a lot more the last couple of seasons, though "what to do with Ann" has been a problem for the show to creatively solve since season one. Chris Traeger, however, is LITERALLY one of the best characters on the show.
Also, it's questionable how much Andy there'll be this season since he's filming Guardians of the Galaxy. (And he's enormously jacked these days.)
Silver lining: If this means more Donna scenes, it might not be so bad.
I adore this show so, so much - but the storyline with Ann and the whole baby thing really has been the weakest part of the show. And her role as 'Leslie's Best Friend' has drastically diminished since Ben came along.
Rob Lowe is a mixed bag. I personally think the actor-guy is a big weirdo-creep - so I was initially pretty against seeing him on my television. But the character really grew on me, and he did a great job taking a minor, secondary role and making it into a pretty excellent part of the show.
I guess my biggest concern is that they've really done such a great job of balancing this little Pawnee universe that I am nervous that any drastic changes could be damaging. They did recover nicely from Paul Schneider's departure, though, so who knows.