There is no traditional full retail version of Windows 8. The nearest that I know of is the System Builder license, which can be used similarly to the old retail license. I'm not sure whether it can be used to do an upgrade-in-place, like the old full retail license.
Normally, an upgrade license requires that an upgradeable older OS be installed. (Some would say
and activated.) Just having disks for the older OS isn't good enough. However, there's another activation method:
http://www.eightforums.com/tutorials/13375-clean-install-windows-8-upgrade.html?filter[2]=Installation and Setup
Yes, an upgrade copy can be used to do a clean install. You can even delete the existing partitions on the OS install drive. It's a bit different than older versions of Windows: even if you delete the OS partitions, some settings and files will be carried over from the old installation.