I’d start with a trawl through EF or similar to get an idea of loadings.
I’d assume the 787 may be difficult to upgrade on ??
Registering for an upgrade for a flight in a few days may have you low down pn the list ?
Do you really want to abandon the security of CX J for a place in the QF lottery ? Does’t make sense to me, but ETTO.

Thanks. This seems to suggest Perth might offer more opportunity for upgrade.
Why would the 787 be difficult for upgrade? Do you mean because no pax will be upgraded out of J into F?
I need the status points so would prefer revenue fare. I can survive Economy if it comes to it (and I realise it may well).