Please read the links; these specifics are addressed in them.
Win10 upgrade is NOT an OEM license; it is only good for one PC ever, major hardware changes make it a different PC.
Thats what an OEM license is.
With the OEM version, Windows is tied to the machine you first put it on or that came installed when you bought the PC.
With the full retail version, as long as you remove it from the old machine, you can then put the it on any new machine you build or buy.
There is no difference in the operating system other than the Terms of Use.
So for sake of upgrading to Windows 10,if you have an activated OEM license then after an upgrade you to Windows 10 that becomes an OEM license good for that machine only.
If you buy Windows 10 retail version it is not hardware/PC specific(OEM)and you can install it on any PC or Hardware configuration because you have a product key and the full installation CD.