your computer’s hardware is registered with Microsoft’s servers. When you install Windows 10 on that PC again in the future, it will check in with Microsoft’s servers, confirm it’s installed on a registered PC, and automatically activate itself.
Get your new licence key
After your computer has upgraded to Windows 10 it will have a new licence key. You'll need this if you want to perform a clean installation of Windows 10, as you can't input your existing Windows 7 or 8 code into the installation routine. To get your key fire up Belarc Advisor as above, let it perform its analysis, and find the product key in the results page - in the current version of Belarc (8.4), it is listed under 'Windows 8.1'. It's a good idea to copy and paste this key into Notepad and save it somewhere safe.