New
#11
Lots of people have done it, but it violates the EULA (End Users License Agreement) to do so. You have 1 license for 1 install of Windows. On an upgrade to Windows 10 from Windows 7/8/8.1 it does not matter if that Windows installation is the original Windows 7/8/8.1 or the Windows 10, but 1 license still is for 1 install of Windows only. You are free to switch between the two, but not have both installed at the same time (nor have multiple copies of the same version installed at the same time).
Also, when you upgrade with the free upgrade - it isn't the license for Windows 7/8/8.1 that gets the entitlement for Windows 10 - it is the computer that was upgraded that gets the entitlement for Windows 10 - but as long as you have Windows 10 installed on that computer, it is using the license from the Windows 7/8/8.1 that it came from to satisfy the requirements of the EULA. The Windows 7/8/8.1 is LEGALLY bound to the Windows 10 install - but not physically bound to it (as in blocked in any way on Microsoft activation servers). Any product key for Windows will get physically blocked from automatic online activation if it is used too many times within a certain time period - then you have to call the Microsoft activation phone number and tell the computer voice that you have that copy of Windows installed on only 1 computer (and that is also true of any Microsoft software product.)