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#11
When a computer is upgraded to Windows 10 from Windows 7/8/8.1, the unique Hardware ID of that computer along with the digital entitlement for Windows 10 is stored on Microsoft activation servers. When Windows 10 is re-installed on that same computer, you skip entering the product key. Once it is connected to the internet, Windows 10 will send the Hardware ID to Microsoft activation servers and the matching digital entitlement for Windows 10 will be returned and Windows 10 will activate itself.
The only part the Windows 7/8/8.1 product key plays in this whole process is it is used to create the initial digital entitlement. The Windows 7/8/8.1 product key is not affected by this process - as in it does not become invalid. It remains valid for use under the conditions of the EULA that applies depending on the type of product key it is, retail or OEM.
The 30 day time limit is only for the rollback feature. A scheduled disk cleanup deletes the files needed to do the roll back. You can still go back to your previous OS after that date. You just reinstall that OS, or refresh your image you made before the upgrade, etc. I did it myself as a test to verify you could do it. I reinstalled Windows 8.1 with a clean install using the same key it was using when I did the free upgrade. It activated without issue. Once you do the free upgrade your PC gets a digital entitlement for Windows 10. You keep that even if your roll back. I did a clean install of Windows 10 on this same PC and did a skip when asked for a product code. Windows 10 installed and activated automatically with the digital entitlement.