If we can ignore enterprise licensing then traditionally there have been two types of MS license - retail and OEM. Both have been tied to the mobo but with the retail version you had the right to move to a different mobo three times with automatic authentication. The OEM version is tied to one mobo and cannot be moved to another mobo or computer. Basically, MS sees a different mobo as a different computer. As far as I know, MS continues this retail/OEM distinction with Win 10.
Since we are dealing with MS, nothing is quite as simple as this. MS does accept that mobos and CPUs can fail and you are allowed to replace these components as a repair. Where it gets 'gray' is that say two years or more down the line you may not be able to find identical replacements and have to use newer components and these replacements are inevitably going to be something of an upgrade. When this happens normally a phone call to your country MS authentication service will get you authenticated and you are good to go. Sometimes the new mobo will authenticate automatically.
I have only had to phone MS once, quite a long time ago, and there was no problem once I got to a real person, I think I was using XP at the time. Almost exactly two years ago I got one warning that my CPU was about to die - which let me shut down in an orderly manner - and then it refused to run again. A new mobo and CPU, reinstall Win 7 (and everything else) and Win 7 just authenticated, and I have an OEM version ! All you can do is try it and see what happens. But if you do have to phone MS, then it was a repair wasn't it ?