NAN-LAX is only USD 1500 one way in business, so you could consider paying for the return. Their "bula" upgrades start at USD 800 but not offered on award flights. At the airport they also sell upgrades. It should be cheaper from NAN but I think it was relatively pricey in 2019 when a family member purchased one on a non-rev ticket.

p.s. If you are based in Fiji the NAN-LAX-NAN return is only about USD 2100. The other direction at least USD 4600.