Digital license

When you upgraded from a previous version of Windows or receive a new computer preinstalled with Windows 10, what happened is the hardware (your PC) will get a digital entitlement, where a unique signature of the computer will be stored on Microsoft Activation Servers. The Windows 7 or Windows 8 genuine license you were previously running will be exchanged for a diagnostics key.

Anytime you need to reinstall Windows 10 on that machine, just proceed to reinstall Windows 10. It will automatically reactivate.

So, there is no need to know or get a product key, if you need to reinstall Windows 10, you can use your Windows 7 or Windows 8 product key or use the reset function in Windows 10.

If you are prompted to enter a product key, click 'I don't have a key' and 'Do this later'

Download the Media Creation Tool Now

Right click the MediaCreationTool.exe file then click Run as administrator.

Note: How to verify if you downloaded or installed the Windows 10 Anniversary Update

Select the option Upgrade this PC now if you want to upgrade right away, of if you want to download a ISO image or create a bootable copy on a USB thumb drive, choose Create installation media for another PC then click Next

A new option has been added to the setup screen for users reinstalling Windows 10 on a previously installed and activated system. If your system has a Digital License attached to it, Windows 10 will automatically upgraded:

The answer is yes. Once Windows 10 is installed and activated the licence is given for the life of the device. Was it Windows 8.1 Pro or did you change from Windows 10 Home to Pro after the upgrade.

Why not use Windows 10 Reset and then you won't go back to Windows 8.1.