Well, I couldn't be bothered to wait for my W10 upgrade over W7 / 8.1 download so I installed W10 from a install USB flash media I created last night. I thought I could install W10 clean. Wrong! I wiped my C drive by formatting and installed W10. I assumed that signing in with my MS account would verify that I was a legit upgrader. However, I knew this was not going to work as W10 asked for a product key. I skipped this and installed anyway.
Major Problem....once installed, W10 could not activate! Nothing would work. So I went back to W8. But I forgot that my w8 copy was also an upgrade!!! So I had to scrub that installation and install Vista Business 64bit!!!! I Good job my PC still has an optical drive! (I did have W7 but 'lent' it to my sister as an upgrade over Vista on her PC).
Unlike W7 / Vista could not natively detect my network adapter. So I had to did out my motherboard's driver disk for the first time in a good three years and install from there. Then activate Vista. Then plug in my W8 USB install flash drive.....wait a minute, Vista dies not natively support USB 3.0 so I had to install in a USB 2.0 port round the back
of my PC.
Once I clicked the install file on the USB flash drive, Win 8 installed as an 'upgrade' after wiping all the old Vista files. Repeated the same procedure with W10 over W8.
So now it is all done.
I do remember having this issue with W8 over W7 and having to install initially as an upgrade. But then subsequent installs were allowed via the old fashioned format and install routine. I guess the same is now true of W10?
P.S. I did read that W10 has to be upgraded over W8.1 and W8 was not eligible. Not true as I went straight to Win 10 from W8 completely skipping the 8.1 upgrade.