Well I don't see why you should have to change all the roles since you're kicking DD out, the person who knows them. Can't you just give his account a new player?

If you change the roles everyone knows at least one of the roles in game. If they were mafia then they could know what roles are there. If you're gonna switch roles switch the mafia within the mafia. It'd be cheating if like a goon got cop. They'd know exactly what to look for.

EDIT: also changed password.