You can book cheap flights with them right into January next year. For example: DMX - HKT for 834 Baht one way.
Also, I've never had a problem changing seats.
Good airline in fact and very accommodating: http://youtu.be/ApUxqIXvNgA
I've never tried to change seats online so you might be right, but I got the impression from their site that they allow seat re-assignments twice: http://www.nokair.com/contents/journey_plan/how_to_select_seat/en-US/index.html