They'd have to do a relaunch with some of the original cast serving as instructors at the Police Academy. Maybe Steve Guttenberg reprises his role and takes a new cadet under his wing to play new practical jokes on Harris. I would think that Jones, Harris and perhaps Proctor would return. It would then mean that it's not necessarily a reboot if some of the original cast return.
I don't think the plan is to have Mahoney played by someone else. I think the plan is to have some of the old characters return and then have the new cast take over, with their own identities and own personalities, rather than have an exact copy of Mahoney, Jones, Hightower, Harris or Hooks. I could be entirely wrong though.
I think it would be like Ghostbusters 3 or the reported National Lampoon's Vacation redo. With the Vacation relaunch the plan there is for Rusty to plan a family vacation and to have Chevy Chase return as Clark Griswold. And most of us know that the plan for Ghostbusters 3 is to have Dan Aykroyd, Bill Murray, Harold Ramis and Ernie Hudson return as Ray, Peter, Egon and Winston and then have them bring in some new faces.
That's how I see this new Police Academy being done. To me a remake is where a movie is redone with the same characters from the original, but with different actors playing the characters in the remake. It would not be a remake in the true definition if Steve Guttenberg, Michael Winslow, Bubba Smith, George Gaynes, GW Bailey and Leslie Easterbrook reprised the roles of Mahoney, Jones, Hightower, Lassard, Harris and Callahan. It would be more of a relaunch.
As for the suggestion of having the Judd Apatow stock of actors in the cast? I don't think it should be Jonah Hill or Seth Rogen. I think it should be a lesser known member of the Apatow crew such as Martin Starr.