Apparently airlines do this occasionally and if you do not have a high loyalty status with them you are first in the firing line for down grade.
Pls see below responses for more details.
Thanks for supporting.
Check in staff at airport ( Not Qantas staff)
Told us the flight had been overbooked and as we did not have high enough frequent flyer status we had to fly in economy.
We said this was not good enough and eventually a Qantas employee came and said we had no choice, there were 3 seats not working...
Qantas is now behaving like Jetstar. We were just bumped down to economy for overnite flight from Tokyo to Melbourne. Ticket paid and booked 7 months earlier, discounted but still not cheap!
As only Bronze status we got bumped.
Will definitely never fly Qantas business again!