What happens if a Windows 8 user wants to upgrade and clean install Windows 8 Pro?

I have a Dell XPS One 27" AIO coming soon and it ships with non-configurable standard Windows 8.

I want to clean install Windows 8 Pro when it arrives, but it seems that's impossible without getting a system builder OEM disk that doesn't come with any support?

All I can find are Windows 8 Pro upgrade options, the first being from XP, Vista, or Seven, and the second being a software upgrade from within Windows 8?

Can anyone please help with this?