You’ve paid the F mileage, but the ticket would need to be reissued for the SIN-LAX sector so it was in F. You’d do this before departure, if award seats were available.
The only aircraft with the range for SIN-LAX are the a350s, so they top out at J.
If they changed the aircraft to a 777 you’d be flying via HKG/TYO/ICN etc.
