Here's a post I made elsewhere at the very beginning of march regarding the situation of Windows 10, upgrades, and clean installs:
http://www.hardwarecanucks.com/forum/o-ss-drivers-general-software/74522-fresh-win-10-install-2.html#post841709
Inputting the Windows 7/8.1 CD key into the Windows 10 installation may actually be seen and activated as a new device. I mention this because some people have been able to use their Windows 7 keys quite recently to install a fresh copy of windows on new hardware. Why is this still working as they stopped upgrades? Ask Microsoft.
Fresh install on a machine that previously had Windows 10:
1) Digital Entitlement. NO Microsoft Account associated. Reinstall Windows 10 and do not enter a key. It will activate via internet based on mobo/cpu ID. People encounter mixed results with this method.
2) Digital Entitlement. Microsoft account associated. You will see the device on your MS account. Reinstall Windows 10 and do not enter a key. It will activate via internet based on mobo/cpu ID once you have logged into your MS account again on the new installation.
Upgrading hardware on a machine that already has Windows 10
1) Install Windows 10 and enter the previous Windows 7/8.1 product key you have. This will create a new device ID/activation association. Mixed results reported on this method.
2) Digital Entitlement. Microsoft Account associated. Make sure your current device has a Microsoft Account logged in and associated. Upgrade hardware. If your device deactivates, you can go to the "Activation" section of Windows and migrate your Digital Entitlement over from your previous hardware setup to your new one.
Note - Logging in with a Microsoft Account will attach itself to the local account that is currently logged in. Do not do this if you use the built-in admin account. Create a separate local admin account first - but really, you shouldn't be using the built-in admin account anyway.
Alternatively, there's an interesting method another individual mentioned just below my post you can use to grab a particular file, clean install, and restore this file to gain your activation back. It was originally used prior to the Nov '15 update that allowed you to directly upgrade Win 7/8.1 to Windows 10, but it seems to still work if you'd like to reinstall a clean copy of Windows 10.
Note - It will only work if your hardware remains exactly the same:
https://www.tenforums.com/tutorials/23354-clean-install-windows-10-directly-without-having-upgrade-first.html
