Many years ago I was on a Lufthansa 737 Istambul to Manchester via Frankfurt and had booked a window seat. It was on the RH side near the front - and there was no window! The disadvantage of a non-aisle seat without the compensating view! The flight on its first leg was lightly booked, so I moved seats.
On arrival at Frankfurt I took my ticket to the Lufthansa desk and asked for a window seat, as originally requested.
"This ticket is for a window seat".
"The seat enjoys no window. Please change it!"
They did and I got a 'proper' window seat for the second leg of the journey which was as well, for that leg was full so the 'move seat' option wasn't possible.