Once you have purchased a license for a unit of a Windows 10 operating system, you are well authorized to use it on the device till the support for the product ends (rather even after that).
However, the Windows 10 license is linked to the hardware of the device. In case you change any critical hardware (eg. the motherboard), the device would fail to recognize the license. Use the following procedure to rectify the issue:
Log in to the system as an administrator. Go to Start >> Settings >> Update & Security >> Activation.
It would read the message "Windows is activated with a digital license linked to your Microsoft account." or "Windows is activated using your organization's activation service." If it doesn't display this message, proceed with the following:
If it is a home computer (or a personal one), log in using your Microsoft account as explained below:
Go to Start >> Settings >> Update & Security >> Accounts >> Your info, and select Sign in with a Microsoft account instead.
Now return to the Activation page and check, your license would be activated.
Yes; Windows has always been activated based off a hardware fingerprint though. Windows 10 has just made that process more transparent and with the most recent build made the process of transferring eligible retail licenses to another machine even easier.
A digital license (called a digital entitlement in Windows 10, Version 1511) is a method of activation in Windows 10 that doesn't require you to enter a product key. If you upgraded to Windows 10 for free from an activated copy of Windows 7 or Windows 8.1, you should have a digital license instead of a product key.
How does a Windows 10 'digital license' activate the OS?
The only difference between a traditional Windows license and a digital entitlement is that you no longer have to enter the license key when you install Windows 10 if you have a digital entitlement. Windows 10 will automatically activate. Additionally, and specifically optionally, is the ability to link your entitlement to your Microsoft Account.
Source: Activation in Windows 10
Thought the disk drive may change, I expect that none of the other fingerprinted hardware will change.
Windows activation status has never been connected to the HDD.
If you have activated a free copy or bought and activated a copy of Windows 10 from the Store, you have a “digital license” (digital entitlement) for that device.
This means that you can reinstall Windows 10, even a clean installation, and you don’t need to enter a product key. If during the installation you’re prompted to enter a product key, select Skip. Then the operating system will activate automatically after the installation.
If you previously installed and activated Windows 10 using a product key, then you’ll need to enter a product key during the installation.