I have done exactly like what you said. I reserved a free upgrade for windows 10. After upgrading to Windows 10. I found that my pc was too laggy and I decided to do a clean Windows 10 installation.

Before clean installation, I used Magical Jelly Bean Keyfinder to retrieve my Windows 10 CD Key and keep it somewhere else. I downloaded Windows 10 ISO file: https://www.microsoft.com/en-us/software-do...ad/windows10ISO

Then, made a bootable Windows 10 USB installer using Rufus.

I did a clean windows 10 installation. During the installation, I entered the windows 10 CD Key I retrieved earlier and it worked and activated.

I noticed that Windows 10 CD Key is different from the original Windows 8 CD Key which I used to upgrade to Windows 10.