Unfortunately you will need to continue working with the Microsoft store customer support as Microsoft owns the store, your account and all licenses attached to it.
http://support.xbox.com/contact-us
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It would help if Bungie/Activision can set straight with MSFT what qualifies you for the digital upgrade path. They have no clue, I called three times and got three different answers to why it wasn't available for purchase on my xbox one. Its very easy to pass the buck back to the consumer (as you did above) that's not a very good way to generate good feelings