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Simple answer is you can’t! (I say this so that someone will take on the challenge and solve the problem)
Can you believe that it is not possible to just buy the Windows 8 Pro upgrade which will automatically upgrade Windows XP/Vista/7 using the Upgrade Assistant and install it when running the evaluation release of Windows 8?
What on earth were Microsoft thinking when they forgot about the thousands of us who upgraded their perfectly working Windows 7 machines to get a glimpse of Windows 8, expecting to be able to simply buy the official Windows 8/8 Pro license when it was released.