For all the RETAIL licenses: after I had read the above comments regarding the retail issue, I called directly Microsoft technical department and they explained me how things will be with respect to the Windows 10 license transfer to another PC or just motherboard upgrade. I have a Windows 8.1 retail license; after I upgrade to Windows 10, I will be able to use and reinstall that Windows 10 license unlimited in time. I will still retain my same Windows 8.1 license, which will not change, so I can revert and reinstall my 8.1 whenever I want. If and when I change my PC components, I will have to call Microsoft technical department: after I prove them that I have a valid Windows 8.1 retail license, they will transfer/activate my Windows 10 free upgrade to the new PC/motherboard and deactivate the old station (as it happens sometimes with OEM licenses, once I called and asked them to transfer my old OEM license because I changed the components, and they were that kind and they did it).
My question was pretty straightforward: will I be able to transfer my Windows 10 upgrade to a new PC/motherboard? The answer was "yes", as long as I can prove that I own a retail Windows 8.1 license.
My second question was: Why is it so complicated and why was not the actual license converted into a Windows 10 retail license, for easier future reinstall? Their answer was that this upgrade is just a free upgrade, and will be treated as such, as an upgrade. When I want to transfer the Windows 10 upgrade to a new system, I will have to install first Windows 8.1 retail on that new machine and after that to apply the Windows 10 upgrade; so being an upgrade means that they will have to manually reactivate it on a new machine each time I do such a modification. Well, it could have been made more convenient, but it's ok like this, hey, it's a free upgrade at least.
I hope this helps.