I found the following FAQ on Apple's website:
Why can't an upgraded version of Microsoft Windows XP or a full version of Microsoft Windows XP that does not include Service Pack 2 (SP2) or later be used for installation?
An upgraded version of Microsoft Windows XP cannot be used because you are required to validate your copy of Windows by inserting an original Windows CD, but there is no way to eject the first disc until after Windows installation is complete and the Boot Camp drivers from the Mac OS X 10.6 DVD are installed.
And so I was wondering if you had two DVD drives hooked up to a MacBook Air, could you install boot camp using an upgrade version of Windows 7?
Best Answer
What are you trying to upgrade it from? If the answer is WinXP, then no. From Apple's 10.6 bootcamp install pdf: