Let me play devil's advocate here a little bit. Note that I am not a lawyer, so everything I write here is just an opinion based on attempting to read Windows license agreements in as much detail as possible. Everyone seems to assume and agree on what appears to be obvious: that when you upgrade to Windows 10, its license replaces the license of the base genuine Windows you upgrade from (7, 8 or 8.1). Hence you shouldn't be allowed to upgrade to Windows 10 on one computer, then reinstall the base Windows (e.g. 7) on another computer (assuming the base Windows has transfer rights), and carry on using both computers. However, to me it appears that nothing in the Windows 10 License Terms actually say so. It further appears to me that the Windows 10 License Terms provide an additional license that is separate from the license of the base Windows you upgrade from and that in fact you retain both licenses and are licensed to run one instance of Windows under each. Let's look at this in detail. If you look at the Software License Agreements for Windows 8 or 8.1, they contain the following text: There is no such language in the Windows 10 License Terms! In fact, the only reference that the Windows 10 License Terms contain to upgrading, in the context of licensing, is in this paragraph: Note that this paragraph only says that if the base Windows you upgrade from is non-genuine, then you don't get this license, which makes sense. Other than that, the license granted in these terms is not conditional on and is not linked in any way to your license for the base Windows you upgrade from. There is no mention in the Windows 10 License Terms of losing the base Windows' license or that this license replaces the base license. There are no restrictions on what you can do with the base license. You are explicitly granted a new license in these Terms, which is in addition to the base license and is separate from it. I understand that this analysis is contrary to what common sense tells us, but when it comes to legal documents, the actual text of the document is what matters and not assumptions. What am I missing?
Has anyone tried this? If this the case then one can install as Windows 10 on as many computers he likes with just one license.
He actually means that if you upgraded W8.1 key -12345 on first machine, can you still use W8.1 key -12345 on second machine to run genuine W8.1
Indeed this is what I describe in the OP, however if this is legitimate, then it follows that you can upgrade to Windows 10 on multiple machines, as per ikith's suggestion. It is indeed relevant whether the base license is OEM or Retail, which is why I said in the OP "assuming the base Windows has transfer rights". Do note that I am not talking about whether any of these suggestions work in practice (i.e. activates or not), so product key is not relevant. I am strictly talking about whether the license allows this, i.e. if it's legitimate to do so. That's a more subtle question. I am beginning to think that the Windows 10 License Terms are (deliberately or accidentally) weak on this point, and hence this is a legitimate loophole.