You may be able to change your flight if U class (Business award) is available on your new flight.
If that is not available and more than 24 hours before the flight you may be able to cancel the upgrade; but that would incur a 5000 point fee:
Frequent Flyer Terms and Conditions | Qantas
Originally Posted by part of 16.1.9
If the Member cancels a confirmed Classic Upgrade Reward more than 24 hours before scheduled departure, the Classic Upgrade Reward may be refunded and will incur a Refund of Unused Classic Upgrade Reward Fee (see the
Fee Schedule).