No one's been able to comment on the mechanics of how upgrades will work. I can only presume that if an upgrade exists at the time you request it, it is fulfilled immediately, otherwise you go on a waitlist. When on the waitlist, fare type will always exceed status (i.e. a Flex Gold will outrank a Choice Platinum, if it comes to that).
Out of BNE, the NZ lounge is used.
It's 69k points to upgrade from Flex; 99k points to upgrade from Choice.
The website is a bit vague about this; I found these sources:
The latter link seems to suggest that you'll have to pay the new prices.
...which seems a bit rough.
Probably worth a call to check, particularly if you're a Platinum.
My gut tells me no, but I'd rather defer to someone else here who knows better.