Royal status seems to remain and it probably should. After all, if you throw out the bums and have a new ruler but don't have a problem with the old-old retired ruler they still command great respect.
You could just give a 60 day window for kicking out Royals after a rebellion, but really, you could drop metric tons of fines on them. It should be amazingly hard to remove a duke who was one a king who didn't declare during the rebellion.
