The windows 10 key is never linked to disk. I've done more than a few HDD to SSD migrations for myself, relative, friends, and work machines. The key is bound to the motherboard. I've swapped CPUs and not trigger the activation, go from i5 to i7, core 2 duo to core 2 quads, phenom 2 to FX-pile_of_______, etc, windows 10 does NOT care. But the moment the mobo is swapped, I've replaced mobos on latops and desktop systems, and that when the key activation issue is triggered.
However in the case of motheboard swap, if you do NOT want to call Microsoft, and you have a valid windows 7 key and you are not transitioning to the newer kabylakes, ryzens or whatever Microsoft has chosen to forbid for upgrades (since they technically didn't exist prior to windows 10), just reinstall windows 7, do the windows 10 update like this article indicates, and I don't believe there should be any issues for your conscience. Mobo swap reusing your old CPU, GPU, SSD, HDD, etc. should be fair game.