I think mstr caution is correct. There are two new domestically configured mainline A330's arriving in the near future and I predict the spare A330 not going to Tokyo will be gainfully employed on the Perth run.
The two new J* Tokyo services are a last ditch stand to hang on to the precious Narita slots until, and if, the the tourist market recovers. If it does then I'd predict mainline would go back in there, if not, then Qantas may ditch the slots and bid all but the QF21/22 service goodbye.