You can make pretty much an unlimited amount of 1-2 changes. You just need to make sure it is linked to your microsoft account. To the best of my knowlege, no hardware change is allowed where windows was upgraded, but if you buy a copy of windows then you get at least one hardware change. The issue lies with upgrades to windows 10 vs buying a copy out right. I've since reinstalled windows and am still fully activated. I had issues with the hardware so swapped mobo and cpu to an amd system. I had an intel system that i had a bought and paid for copy of windows for. If you are connected to the internet your issues with activation will be fixed

If you still have the key # lying around, go to Settings -> Update & Security -> Activation -> Change product key, and insert the number. If you transfer a drive with a W10 installation that was activated with a physical key, then transferred that drive to a another computer with very different hardware IDs, then Windows flags it as a illegitimate copy.

Speccy cpu temp all over the place