Why do you think they sell tickets in booking classes F & P?
If purchased today as part of a roundtrip ticket, SAN-MCO on 7/20 would cost $365 in coach or $525 in first, a difference of $165. Next time, plan ahead & cough up the extra $160 and you will have that confirmed seat in F.
For people who don't plan ahead and want to change their itineraries prior to departure, they have change fees. You agreed to the terms and conditions of your fare class (including the change fees) when purchased your ticket.
You can even do it yourself on nwa.com:
Reservations Center
View/Change Reservations
select itinerary
Change or Upgrade Reservation
Change reservation - re-schedule flights
It will walk your though the whole process and you can end up with your "paid upgrade" in F.