You can call and move the date of an upgraded domestic flight, retaining the upgrade if the new day has availability.

Did this today, stiff change fee plus assisted booking fee almost $100, plus big up-fare as today is between-sales-day for QF domestic. But still worth it to retain the J seat.

Alternatively you can cancel the upgrade online by selecting "upgrade" in "your bookings", there is a cancel button. It has to be done manually and takes 1-3 days. Once that goes through and you get the points back, minus the 5k cancellation fee, then you can cancel/change the flight online.