I think they'll get back the ones who always come back. I'd be shocked if we get a new Mitsuru, Yukari, or Akihiko. But yeah some of them have already been recast in later games, namely Fuuka and Ken. And Atlus isn't opposed to game-wide recasts. P4G wasn't even a remake, just a port with new content, and they completely redubbed Teddie and Chie through the whole game because the old actors didn't come back for the new lines.
As for Junpei and Aigis... I think Junpei getting recast sounds like a good bet. I hope so, at least. Aigis... I'm less confident on that one. Her views weren't as huge a news story as Vic's crimes and it's possible they won't care as much even if they do know about them.