To qualify for the cheaper Windows 8 upgrade
copy instead of a more expensive full or system builder copy, you would need to already own either XP SP3, Vista, or Windows 7.
When you install the Windows 8 upgrade you are agreeing to the EULA to no longer have the specific product key of the qualifying OS (XP SP3, Vista, or Windows 7) activated for the lower price. If you uninstall the Windows 8 upgrade, then you can use the qualifying OS product key again.
If you can still have say Windows 7 installed and the Windows 8 upgrade installed at the same time if you have another product key to activate with other than the one you used to qualify for the Windows 8 upgrade. You could also purchase a more expensive system builder copy of Windows 8 to be able to keep both Windows 7 and Windows 8 installed and activated at the same time.
It's like this because you can't have your cake and eat it for the price break on a Windows 8 upgrade VS more expensive full copy.
Hope this helps to clear it up more.