Christopher Elliott takes on his role as the world’s worst travel advisor and tells his audience in USA Today that they have his permission to take any empty seat in coach since flight attendants “can’t police every seat.”
By the way that’s one of his “tips for getting an upgrade.” So presumably we’re not even talking about moving between regular coach seats. Just sit in the premium seat and if questioned tell your cabin crew (and anyone they call in for reinforcements that you had Christopher Elliott’s permission.