Microsoft have changed the way activation works
Reactivating Windows 10 after a hardware change
In Windows 10 (Version 1607 or later), you can link your Microsoft account to the Windows 10 digital license on your device. This can help you reactivate Windows using the Activation troubleshooter if you make a significant hardware change later, such as replacing the motherboard.
But we don't know how many parts can be swapped and still keep it activated as they say later
If you’re signed in using the correct Microsoft account, here are some additional reasons why you can’t reactivate Windows:
- The edition of Windows on your device doesn’t match the edition of Windows you linked to your digital license.
- The type of device you’re activating doesn’t match the type of device you linked to your digital license.
- Windows was never activated on your device.
and its the bottom one which I think you fall into, but we don't know as this only began to apply on the 5th August this year (a few days ago).
you need to read this to link your current install to the email address you gave Microsoft originally:
Re activating Win 10 after hardware changes
How does this effect you? Well, I would backup anything you cannot afford to lose, and just put old hdd as it is right now into PC and see what happens. It might activate, it might not, it might not even boot due to other fun detail, brand new hardware everywhere.
most of time it won't boot but you should still link your digital entitlement to your email address as that is worth trying once win 10 installed. It may not work but its worth a try. Would save you the money for a new copy of windows.
You can download win 10 from
Here and use that tool to make a win 10 installer on USB or DVD
change boot order in BIOS so it is USB first, hdd second
boot from installer and follow this guide:
http://www.tenforums.com/tutorials/1950-windows-10-clean-install.html
when you reach screen asking for licence, click "Skip" and win 10 will install. Then you need to run that troubleshooter mentioned in the Re activating win 10 after hardware changes link and see if it works. If it doesn't you have 30 or so days to buy a new licence.