If you have a copy of windows 7 that purchase between June 2, 2012 and January 31, 2013. You are eligible to avail the 19.99 discount copy of Windows 8, on Philippine currency that’s almost 819.59 Pesos. You can go to (http://www.windowsupgradeoffer.com/) and fill up the required form then add your Windows 7 Serial Number. The regular price of Windows 8 is at $39.99 you can buy it at (http://windows.microsoft.com/en-US/windows/buy).
On my situation, One of my friend asked me to reformat his PC, he bought it a couple of months ago and I asked him for the exchange of reformatting his PC, I will use his windows 7 serial to avail the Windows 8 discount. he said OK and BOOM I got windows 8 at $19.99 instantly.
You can also avail the Two back up disk that comes with Windows 8 32 bit version and Windows 8 64 bit version for another $14.99 and it will deliver straight to your doorstep. Below are the screenshot of the Disk Kit.
On upgrading your Windows copy, you can also used non-genuine copy of Windows 7 in order to upgrade it to Windows 8.