I think that any sort of miles upgrades out of SYD are few and far between at the moment and this situation will probably continue until there is another SYD/DXB flight added to the schedule. I am travelling in J(flex) from LGW to SYD in two weeks - booked back in June, and have been trying to upgrade to F using miles ever since and it looks highly unlikely that upgrade will happen as F is now pretty full on all legs, having been wide open at time of booking. I will keep trying and might get lucky at check-in but I have never had this problem before Covid, but then there were three daily flights instead of the two at the moment. I have called many times but not yet found an agent who can help.