I just had another look at your first post, and you put you have a retail version of Windows7, in that case you can use your free Windows 8 on any machine you buy or build as long as your windows 7 is not installed on another machine.

I got a retail of Windows 7 and updated to a retail version of Windows 8.1, so I can replace every component in my desktop and still be able to install the free version of Windows 10

You can not do it with OEM


have a look here, took me ages to find it mind you.

When I upgrade a preinstalled (OEM) version of Windows 7 or Windows 8/8.1 license to Windows 10, does that license remain OEM or become a retail license?
If you upgrade from a OEM version of Windows 7 or Windows 8/8.1 to the free Windows 10 upgrade this summer, the license is consumed into it. Because the free upgrade is derived from an OEM base qualifying license, Windows 10 will carry this type of licensing too.
If you upgrade from a retail version, it carries the rights of a retail version.