Passengers can move to where they wants as long if they want to nap, they move to a 3 or 4 seating. sometimes i move without permission, and they know I wants to nap or get better view down to earth. Be sure that no seats are being taken, sometimes plane leaves with less passengers. I was on AA105 on Oct 27 from JFK to DFW, on a MD Super 80... the plane was half filled with pax, and I moved to a 3 seater, and slept 2 hours way down to DFW after the 1st 2 hours reading a book.
they may not think you a hijacker....don't worry & just ask nicely