So, Forza Horizon 2 is released in Aus after the date in which it is released in the US. I’m wondering if I purchase it digitally (which I already have in the Aus store) and then set my region to that of the USA will I be able to play said game earlier than in my own region?
Nope , only if you brought it off the US market.
No because the game is still set to the purchase region.
If you changed region and purchased the game using US credit you would be able to play it early and still on a AU account. But you cant change the region just to get the game earlier as far as im aware, due to the way game purchases are handled by the microsoft server (they are technically listed as different games)
Could you? Yes…
Is it advisable to do that sort of evasion? No.
Just have patience and wait for the street date in your region 
This idea ----> bad
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