You can also try calling QF instead of EK and ask to be rebooked on QF metal A380 SYD-SIN-LHR, if you have a high enough F fare then they will gladly do that -- I've done it before EK to QF change circa 6 years ago, precisely for the same reason (avoid downgrade). The tricky part is that QF cannot make the change even though QF issued ticket, as Expedia controls the booking, but if QF is willing then certainly possible. Getting EK to change your routing is much less likely as they still have good DXB-DUB service. Downgrade compensation from discounted F tends to be close to $0, I've had quite a few on AONEx tickets.