Wìndows 10 full format without losing license [duplicate]

Unless you change to new hardware (motherboard), you should not need to do anything. You should be able to clean/reformat your harddisks and reinstall without an explicit windows 10 key (leave the windows key field empty during the install).

To be safe, I would recommend that you link your windows 10 product key with a Microsoft account. This should allow you to login with that account to re-verify your Windows 10, even if you have new hardware.

Step 1. Make sure you are using a Microsoft account:

Keep in mind that if you did not already have a Microsoft account, that your new login information will change to the email address/password associated with the Microsoft account.

1. Open "Settings"
2. Select "Accounts"
3. Select "Your info"
4. Select "Sign in with your Microsoft account instead" link.
5. Fill the information required
6. "Sign in" and follow the remaining instructions.

The next step is to verify that your microsoft account is associated with the windows key.

1. Once again, open "Settings".
2. Select "Update & security".
3. Select "Activation" on the left bar.

If everything is good, it should say next to Activation: "Windows is activated with a digital license linked to your Microsoft account".

When you now reformat your machine, simply login with your Microsoft account.

Step 2. Hardware Change

As the others have mentioned, if you have changed motherboard you would lose the "windows key" associated with it. That is why you will need to notify Microsoft about "Hardware change".

1. Select "Settings"
2. Select "Update & security"
3. Select "Activation" on the left side.
4. Select "Troubleshoot". It should say something about it failing to be activated on your device.
5. Select "I changed hardware on this device recently".
6. Login with your Microsoft account (if you haven't already).
7. Select the device you are using now, and select Activate

Sidenote: I have recently changed my own hardware (motherboard, CPU, GPU), and followed the steps above in order to keep using my same Windows 10 key. If for some reasons you had to take additional steps; please comment and let me know so I can update the step-by-step answer :)