Something to remember
I seem to learn things the hard way most of the time so hopefully this will help someone out.
In order to use your older-version windows key (7, 8, 8.1) you first need to do the "upgrade" option from your old software, verify that your windows 10 copy is activated, then you can do your clean install.
I didn't do that - I booted from the flash drive, formatted my 250gb SSD drive, & it just wouldn't accept my code. So I skipped it, finished the install, & it still wouldn't accept the code. It was happy to offer a link to buy a license for $199....which I considered for almost 10 seconds.
So I re-installed win7, activated win7, rebooted and then did the upgrade from win7 to win10. I tried to do a clean install (format the drive again) but it wouldn't accept my key.
I'll probably just run it like this. I'm kind of kicking myself...and no, Windows 10 is not FREE in any stretch of the imagination. Without an activated copy you can't even change the display resolution.
I'm kind of kicking myself for even doing the update but now is as good as time as any, I guess.