I have recently had problems with desktops falling over with mobo failure and a heat sink breaking its retaining clip.  All were on ageing equipment so no hesitation in replacing them.

On the first machine I had slightly younger desktop donated to me from my wife's employer.  It came with Windows 7 pro and configured for a corporate network.  I burned a Windows 10 ISO (included the anniversary update) to a DVD, booted to the DVD and installed Windows 10 pro.  Once installed there was a windows.old directory (presumably the old 7 pro) but 10 was not activated.  I found a link to MS Support, got the call back and for the price of allowing them remote access they activated Windows 10 at no cost.

For the second machine (running Win 10 pro) I bought a new desktop with mobo, processor and memory (only).  I put in the dvd and hard drive from the failed desktop. When I powered up I was surprised that Windows 10 reported as activated.  Everything running as it did on the old desktop - no hours spent re-installing Windows and all the apps.

So all good, but I am surprised that I could achieve both.  While I am not an avid reader of MS EULAs I was certain that I was in violation over what I had done. Evidently not.