Ralph Fox wrote:
Windows 7 SP1 and Windows 8.1 Update required.

The above reference from MSFT applies to the GWX Windows Update route

If using media created or mounted by the Win10 Media Creation Tool then
the requirements are not as limited.
- Win7 RTM, Win7Sp1, Win 8.0, Win 8.1, and Win8.1 Update are all
acceptable

The difference being:
- the GWX app using Windows Update is only capable of use on 7Sp1 and
8.1 Update while the Media Creation Tool media accepts all 7 and 8x builds.

--
....winston
msft mvp windows experience