For international flights Qantas do not have specific published codes for "Upgrades". If you are booked on an eligible fare you can waitlist an upgrade request. These are assessed ~20 hours before the flight. Those requests of passengers with the higher QFF status get priority.
For domestic flights U class is employed for both Business class award bookings and upgrades to Business class from Economy.
Now for international flights, it may by that Qantas use the award categories for upgrades at < 24 hours. As upgrades can no longer be requested at that time, this is conceivable.
These award classes are:
- P - First Class
- U - Business
- Z - Premium Economy
- X - Economy.