These situations occur frequently, It could be a mistake on the passenger, seat poacher, someone that wants to seat next to a friend, desire to be close to the front, whatever. Hi, are you 8C, and show him your boarding pass, wait for reaction, if passenger does not move, then bring it up to their attention, or as I have done, ask for $57 which is what I paid to select the seat, they will immediately move. As they do so, just mumble a bit out loud to your self, "why can't people wait till the end before switching seats"