Since Microsoft introduced Windows XP's activation feature, Windows has taken a hash mark of your hardware, everything from the CPU to power supply, to get your PC's unique fingerprint for the activation process. Microsoft has always said it doesn't keep the information, but that may have changed with Windows 10.
It stems from the bargain of having a free operating system. Windows 10's license lasts for the life of the PC and is not transferable to other PCs. But changing just a motherboard can qualify as a new PC.
The first complaint appeared on the Anandtech forums, when a user replaced the motherboard in a Dell PC with another Dell motherboard. Despite having the same BIOS, the user was told Windows was deactivated and the registration key is invalid.
We know that by default, pre-final versions of Windows 10 builds were pre-keyed, meaning you do not have to enter a product key during setup. The key was saved in the Windows Store as part of the new delivery system for Windows 10.
Also, UEFI and BIOS contain a software licensing description table, or SLIC, a digital signature placed inside the BIOS by the OEM. This is used to authenticate licensing to the OEM Windows Installation disk. Again, it knows the hardware, and a change will be recognized as a new system.
Finally, How-To Geek reports that whenever you install Windows 10, a hash generated by your hardware will automatically report to Microsoft's activation servers. Microsoft will confirm that the PC with that specific hardware configuration is allowed to use Windows 10, and it is automatically activated.
Fortunately, How-To Geek points out a tweet from Microsoft's Gabe Aul, who said to contact support from within Windows 10, explain the situation, and they'll activate Windows 10 for you.
This isn't all that different from XP's nuisance activation, and in some ways easier. I ran into activation problems with XP after too many reinstalls on the same PC. That was the good old days of XP, which would progressively degenerate over time. Except with XP, you had to call into a Microsoft line and give a 36-digit confirmation code and then get another lengthy code back, which you had to type in and not screw up.
So, just keep this in mind. If you make a significant change to your hardware, you may end up with a deactivated Windows.