I have always built my own systems. I put an OEM copy of Windows on an early build because I didn't know it would be forever tied to that particular computer; I got lucky and nothing happened. I was ready to upgrade to a new build and started over, upgrading the motherboard and RAM, but using "leftover" parts for the rest of the build. This time I put a Retail copy of Windows on. That way, if something happens and I have to replace a major part, a call to Microsoft licensing, explaining why the "new" computer, will be the only thing I'll have to do.
I no longer build my own systems; I buy them off the shelf, so to speak. These systems (1 desktop and 3 laptops) all have OEM copies of Windows on them because they were built by an OEM manufacturer. One of these laptops is a replacement for a laptop that crashed and burned due to an irreparable graphics "card" malfunction. I bought the computer with Vista installed. If Vista had still been on that computer, I would have lost my license, because it was an OEM computer. Since I had replaced Vista with a retail copy of Windows 7 and then upgraded to a retail copy of Windows 8.1, I could still use the license to Windows 8.1.
Am I making sense so far? Since the Windows that was on the laptop that crashed and burned was a retail copy, I can put it onto another computer if I wish. However, since the replacement computer already had Windows 8.1 installed, I now have an extra copy of Windows 8.1 retail.
If you have a self built computer with a retail copy of Windows 7 or 8.1 on it, I believe the license will be the same as if you had bought the computer with Windows 10 installed by an OEM.
We don't know yet, but if Retail Licenses are available, I would expect that the same license as always would apply. Do I know this for sure? Absolutely not, because Microsoft still hasn't given us all the details.
This is still a guessing game and pretty much an educated guess, but still a guess.