Policy should, at minimum, be to compensate the fare difference between the lowest Y bucket. I don't understand the logic behind the different only being from full-fare Y/Y+ (other than mimimising monetary loss for QF and maximising further annoyance for the pax). The pax derives zero value from a 'flexible' ticket at this stage, and would not logically buy from this fare bucket, they are all ready being flexible enough courtesy of a miscalculation by QF's revenue management.
It's even more illogical as those full-fare paying J pax will often not return.
I'm on QF21 in J early next year and this type of remediation makes me nervous. Luckily as the flight is ex-Aus you have rights which are not limited to those stipulated in QF's one-sided terms and conditions.