"i have searched and searched to find an answer and i think it's not helping.
Doing an AONE4 leaving December Europe-Australia-Asia-US
which FFP should i use?
i live in Australia and UK,
i'm doing approx 50K miles on this RTW
should i collect on QF till the changes on Dec31st (for the upgrade coupons)
then do a AA plat challenge and use that from Australia onwards?"

Doing a 1st class trip you would easily get QF Platinum status and a few upgrade credits which are well worth having, especially since QF upgrades can be used on any paid ticket, even cheap consolidator tickets. Also, if you are passing through LHR at all whilst in status, you will have access to the BA Arrivals lounge

While you have QF Platinum status is it really worth doing the AA Platinum Challenge since you'll have a higher status already? Even with the program changes, mileage accrual on QF will be at 100% +50% bonus plus status bonus for all your 1st class flights. I would suggest waiting till you need a high tier status in a hurry in the future rather than doing th challenge now

Dave