They become available for complimentary selection at check-in, though may appear as blocked on the online check in interface depending on point of departure. KF*G should have no issue asking for free assignment at the airport. For what it's worth I thought row 31 on the A388 was rather cramped and it was unclear if it indeed had more legroom compared to other seats (pronounced limitation of bulkhead vs ability to stretch legs below seat in front). Very different from the unlimited space on the B77W window pairs. The screen placement is also uncomfortably high. I would not pay for these.

AeroLopa seems to suggest window seats in rows 32 and 33 have some extra space due to fuselage curvature. I'd pick those if possible and was quite pleased with 32A on my last A388 flight.