Windows 8 Pro Upgrade but no previous version of Windows installed
I have a legit Windows 7 installed on my C:\ SSD drive. I have about 10 gigs left of 80 gigs total. So I want to buy a new SSD drive and install Windows 8 on it.
Of course, I would prefer to pay only the upgrade price instead of the full price.
Is it possible? How? Is it legal?
 I want to emphasize that the clean install is done on a new drive (I don't know if this change the outcome)