As topic says I allready bought the ultimate edition at full price on the 18/9, and now see that Forza 5 VIP members get a discount.
Have I lost the money because I bought the game early, that hardly seems fair :(.
Contact Microsoft support. Only they can help you.
I bought the game the day the preorder was available for download and I saw the discount reflected in the price of the ultimate edition.
Contact Xbox Support, like Mitch says.