These preferred seats, if not selected/purchased already, can be purchased during the booking process. And yes, available preferred seats will be available during OLCI.

The other thing you can do, since AC seems to NGAF, is just take a free preferred seat after the seatbelt sign has been extinguished, or simply ask a Y FA if you can take the better seat and 99% of the time they won't bat an eye and let you do it.
Because they DGAF.