Za PC Gamer:
http://www.pcgamer.com/how-to-prepare-your-pc-for-windows-10/
Says Microsoft:
Once you’ve upgraded to Windows 10 using the free upgrade offer, you will be able to reinstall, including a clean install, on the same device. You won’t need to purchase Windows 10 or go back to your prior version of Windows and upgrade again.
You’ll also be able to create your own installation media like a USB drive or DVD, and use that to upgrade your device or reinstall after you’ve upgraded.