Retiree with no FF status. but 750k miles on UA before retiring. Travel once a year domestically cross country to visit relatives on East Coast. Always fly economy booking far in advance and pay extra for economy plus with aisle seat due to lavatory needs and mild claustrophobia. Last two trips my seats were switched to non-aisle (still in economy plus) on the long leg of return flights when I checked in online the day before return departure. On both occasions the airport check in staff at the first smaller airport claimed not to be able to do anything as seats were under control of the staff at the airport for the final long sector of the flight to the West Coast. Of course, flights were fully booked and gate staff at the long sector departure airport said " you can ask someone on board to switch". In other words my problem and not theirs. Complaints in the first case to UA online customer support afterwards got a $50 credit towards future flights and in the second case at my lengthy persistence a senior manager at the initial airport made the change back to the original seat manually. At the final airport the change back to the original seat stuck even though the "seat thief" had a duplicate boarding pass for the seat. I want to be able to avoid this problem on my flight later this year (or at least minimize the chances of re-occurrence). Any suggestions?