This is difficult to ascertain as I can only see how many seats are available for sale through EF. Close in, one would could expect that a few of the seats have been sold so it is not as reliable. I have checked every day between 1st November and 29th March and the maximum is F6 (a lot more F6 are seen in the later dates). After 29th March it becomes F0 every single day despite still being listed as a 77W which is probably because they are not sure if they will switch to a 787 for the southern hemisphere winter.

This is contrary to other routes:
CDG - F7+ seen until end of March 2024 (but then goes to max F6 for the rest of the year interestingly)
ORD - F7+ seen until September next year (end of schedule)
LAX - F6 max seen all year (however given the popularity of the route it could be that F6 in the near term is due to seat sales. So it is not possible to see a switch date)

It could be that JL is considering LAX and SYD as the next routes for the A350.

Assuming that LHR will go daily, it could be that JA08WJ is used to service SYD daily until JA09WJ is delivered and then the 2 aircraft will then start covering the LAX flight.