Looks like they do allow to purchase a different seat after check-in assignment. Wierdly, the site shows some of my seats as available, hope they are only available for me, not for everybody else

P.S. Interestingly enough, there is a big discussion on Russian sites right now about one of their lowcoster "POBEDA" assigning totally separate seats for people on the same reservation in order to force them to pay for seat assignment. It all started with a story of FA reporting a passenger to the police for trying to move to his family once the flight boarded...