How hard is it to upgrade my Boot Camp Windows 7 to Windows 8?
I suggest, to err on the side of no problems, that you download the upgrade assistant for your version of Windows. As long as it's Windows XP SP3, Vista, or 7, then you can download the upgrade assistant for Windows 8 Pro while you're using Windows, and the upgrade assistant shouldn't cause any problems. The upgrade assistant will keep your files, documents, and settings for Windows 7 and documents and settings for Vista and XP. The upgrade is also cheaper than buying the disk of it.
There should be no problems in upgrading Windows on Bootcamp.