Take what you can now. You can always cancel and change them to Joker if you feel so inclined later.
Why?
1) Joker requires A class availability AND fewer than 5 seats having been released for Joker purposes for the cabin. It might therefore not work. I have a very high Joker failure rate, although SIN/SYD probably don't help.
2) Some change of plans may result in your not becoming GGL
3) You may have better use for a Joker in the future.
4) You can just cancel a booking you make now for £30 if you find availability later, under a Joker or otherwise.