At the moment the airbus service is regrettably very popular with the customers out of Tokyo reportedly. This situation has to change if the overall objective of getting Qantas out of Japan is to be achieved.
Previous attempts to run this service down so JQ can have a clear run at Japan have so far failed.
In this light pulling a superior cabin product off the route makes sense. No doubt the oldest and most run down of the B744's will service the destination. And as a bonus, when the load factors are not there to justify a -400 level of available seats, management can trot out the old chestnut about Syd-Nrt being a significant loss maker which Jetstar will have to go and save.