Let's say I use the offer for a free Win10 installation before 2016-07-29, using an old Win7 key.
So far, so good, I now have a working Win10 on my PC.
Can I reinstall Windows 10 on my PC after upgrading? Yes. Once you’ve upgraded to Windows 10 using the free upgrade offer, you can reinstall or perform a clean installation on the same device. You won’t need a product key to re-activate Windows 10 on the same hardware. For more info on how to create your own Windows 10 installation media, go to the Microsoft software download website.
What is "the same device"? I have the habit to update my PCs piece by piece; will I have to buy a new license when I replace the mainboard/CPU/video card/HDD?