New
#1
brief Win 10 install from ISO notes
While I had a 10240 ISO prepared from the ESD from the Insider update, I downloaded a new .iso from Microsoft.
The new one is 3.255 GB in size. The older one, 3.899 GB. (Both are Pro X64.)
I re-installed 8.1 from an image. The first time I ran the upgrade, I chose to keep files and settings only. The upgrade went OK, but I could not activate the result. I got the message that the key was not the correct one.
I tried again: restored 8.1, and upgraded it. The result was activated.
For the acid test, I booted from a USB thumb drive with the updater. I deleted the partitions on the boot drive, and clean installed Win 10. The result was activated.
Aside from the detail about needing to keep everything when running the upgrade from 8.1, everything went as advertised.
Yes, you can do a clean install of Win 10 from the free upgrade after once activating the upgrade over an activated qualifying older OS.