Fromthe Change Product Key option on the Control Panel System page, have them enter the Professional key to do an in-place upgrade; then (when the process is complete) have them enter the Windows 10 Enterprise key to perform a second in-place upgrade.
UPDATE Previously, Windows 7, Windows 8.0 and Windows 8.1 users needed to have either of those versions of Windows installed and activated in order to qualify for the free upgrade offer. For persons who needed to perform a clean install of Windows 10 from the outset, it was a two step process of first validating the machine through the upgradeOqd8.