My son is travelling with a friend, discount Y no status, 11/30 JFK to SEA on a 738, 3-3 configuration.

The only seats now available are middle seats, so they're holding two in adjacent rows. I understand that "Preferred Seats are available for purchase as early as 24 hours prior to departure with prices beginning at $4 USD per flight." Chances are there are still two unreserved seats together somewhere in the Y cabin.

They don't want to pay for Preferred Seats. At what point do those seats become available to such ordinary pax? Never? If so, does the reshuffling to grab empties happen before or after takeoff? Thanks.