Read more at source:People who bought a Windows 7 PC on or after June 2 can upgrade to Windows 8 Pro for just $14.99, but they now have to supply the Windows 7 product ID.
Microsoft kicked off registration for the $14.99 deal in August. Anyone who bought or will buy a new Windows 7 PC between June 2 of this year and January 31 of next year can upgrade to Windows 8 Pro for that special price. Shelling out the $14.99 lets you download a copy of the Windows 8 Upgrade Assistant, which helps you install the new operating system.
Cheap Windows 8 upgrade may now require your Windows 7 key | Microsoft - CNET News