Sorry to hear that... Same thing happened to me last week: Booted from Paid Preferred Seat?.

Wife and I got booted from paid preferred (exit row) seats after checking in. I overheard the guy who displaced us telling the flight attendant he booked last minute but has DM status. Phone agent reiterated the “seats aren’t guaranteed” spiel and that we may be moved in certain circumstances, but that it shouldn’t happen just for an elite that happens to want that seat (though this isn’t my first experience where it has). Got a refund and customer service gesture, but I’m not keen on paying extra for specific seats anymore.