I hope this works for you, but I doubt it will. If you've already pre-loaded/downloaded the game from the US Marketplace, that game is tied to the US Marketplace rules/settings (English Language/Midnight Pacific Unlock/Whatever else). Changing your region isn't going to suddenly change the unlock, just like changing your region to Japan isn't suddenly going to replace the language/subtitles in the game with Japanese ones, even if the game page in the Japanese store says "Language: Japanese" - just like this game page now says the early unlock time. Does that make sense?
I think a safer bet would be to uninstall the preload, hard reboot, change region to AUS/NZ, go to the AUS/NZ marketplace page for Destiny, hit manage, install, and you should be downloading the AUS/NZ version of the game, which I believe has a better chance of unlocking early.
Maybe I'm wrong and it's a lot of extra work for nothing, in which case I apologize, but that's what makes the most sense to me.