Obviously there isn’t a rule that says that, it’s just basic common sense. When has a coordinator ever stepped up to be a HC, then got fired after a year and immediately returned to their old role? It’s never happened because it’s a stupid idea, why would you force Nielsen and Richard out for a guy who’s just led you to a terrible season? Not to mention, DAs credibility would be hugely damaged and I can’t see how any player on defense could take him seriously. Absolute non-starter.