Bootcamp upgrade win 7 to 8 .

Yes, it can but a clean install is often best.

 

The Win 8 upgrade installation wizard will offer to perform what is effectively a clean install if you want it. Once the wizard has authenticated a valid Win 7 installation, one of the installation option will be to backup the existing Windows install (/Windows and /Prog Files (x86) and Prog Files to a Windows.old folder, and then do a clean install to c:\windows).

Once installed you'll have a vanilla install of Windows 8 requiring drivers, applications etc to be installed and all of your old Win 7 installation under Windows.old.

http://windows.microsoft.com/en-us/windows-8/restore-files-upgrade-windows-old

I took this exact route on Windows 8 launch day last year, upgrading my Windows 7 Home Premium installation in Boot Camp to Windows 8 Pro. Be sure to have the Boot Camp 5 drivers to hand for when you get Windows 8 installed http://support.apple.com/kb/DL1638