This is called 'cabin roll' and is done all the time at CX

i.e. suppose a plane had 300 seats and the # of bookings in total across all classes are 291. CX can (and tends to, especially close to departure) still sell across all classes i.e. J9 C9 D9...Q9 O9. So they just want to sell 9 more seats and don't really care what fares those would be. If all 9 new bookings are economy, then they'll roll over the excess cabin by cabin upward.