My husband and I booked a Turkish Airlines itinerary from Seattle to Istanbul to Athens, and return, about nine months in advance. We paid extra specifically to sit together in adjacent business class seats (5E and 5F).
At check-in, my seat was unexpectedly changed from 5E to 1K, although still in business class. We tried to resolve it with the airline but were not able to get it fixed.
Fortunately, the passenger who was assigned my original seat kindly agreed to switch so my husband and I could sit together. This mattered to us because my husband has hearing difficulties, especially with accents, and I often need to help him communicate during travel.
From what we understood, the change happened due to a seating issue on the original aircraft and a later aircraft swap, which freed the seat back up.
I really appreciate the kindness of the passenger who agreed to move, and I was grateful for the positive outcome despite the inconvenience.