It'll depend on the license you have with Windows 10. If you have the free upgrade it registers off your hardware rather than a key. Supposedly you can contact Microsoft and explain you just swapped motherboard and they'll reactivate windows. If you have retail version, it shouldn't be any problem activating itself. OEM falls kind of in line with a free upgrade to an extent. Technically they're tied to the hardware the same way a free license is, so you might get away with talking to MS as well. Heres a decent artificial explaining it.
http://www.howtogeek.com/226510/how-to-use-your-free-windows-10-license-after-changing-your-pc%E2%80%99s-hardware/