Well. I had a Windows XP just for gaming and have now upgraded it to Windows 8 for Assassin's Creed 3, because Ubisoft wouldn't let the game run on XP.
All I did was buy an upgrade-disc and let it run on BOOTCAMP. I would advise you to set the startup disc to Bootcamp, because the computer has to restart several times.
All in all it's an easy process
i did it from windows digital install
windows 7 ultimate 64bit to windows 8pro 64bit