Originally Posted by
daniellam
In general, you can always pay the fare difference between what you paid and the fare for the class of service you can fly on. This needs to be done at the "ticketing" counter at the airport.
Check-in / gate agents generally don't know how (or are authorized) to do fare calculations.
It might be better if you do it at the time of booking.
This describes changing or upgrading a ticket. I think the OP was asking about gate/counter upgrades that some carriers offer which I don't believe OZ does.
Most US carriers offer such upgrades but the product & service is usually quite a bit poorer than the better Asian carriers. Still great to be in C/F than Y on a TPAC or TATL route.