From TechRepublic:
Owners of Windows 7 and 8.1 will be able to upgrade to Windows 10 for free but can they keep using that copy of Windows 10 if they need to reinstall Windows or replace their PC?
Reinstalling an upgraded version of Windows 10 on the same machine will be possible without having to buy a new copy of Windows, according to Microsoft.
People who have upgraded to Windows 10 will be able to download media that can be used to clean install Windows 10 from USB or DVD. There will be no need to purchase a new copy of Windows 10 provided it is being installed on the same Windows 7 or 8.1 machine that was upgraded to Windows 10. The installed version of Windows 10, Home or Pro, will be the same as the version they upgraded to.
http://www.techrepublic.com/article/windows-10-will-microsoft-charge-you-if-you-need-to-reinstall/