Assuming that fare bucket number is correct, and you have an upgradable fare (not red-edeal) then a U3 shows that you will be able to apply for and get an upgrade. (assuming no one else steals the seats before then)

Being domestic, it will be confirmed instantly.

I've done this recently - we shopped for flights based on U class availability, then booked an S class fare and upgraded it 30 minutes later.