Koru Flyer - although there's nothing to stop you writing in to complain I'd make the following comments.

64A could well have been broken when the flight came in and they moved you to 64K (as others have said and I would agree, much better seat than 64A anyway but that's beside the point). They gave you a seat of equal comfort as you originally had.

Two things could have happened subsequent to your move. Firstly the seat was still broken and the lady who was ultimately sat in it could have been told of that and asked to be seated there anyway.
Or at the last minute they managed to fix it and subsequently assigned it to her.

Neither you or any one of us can say for sure. I think if you write in you'll find that one of the two things above were what actually occurred.