Two things
It is possible you won't need a new licence to install win 10 on a new motherboard as they have changed how activation works, it can be linked to your email address instead now. You no longer have a licence, you have a digital entitlement to windows 10 on that PC or your user. There is a troubleshooter you can run on win 10 once its installed that helps you with reactivation. See
http://www.windowscentral.com/how-re-activate-windows-10-after-hardware-change
Secondly, when you upgraded to win 10 using your win 8 retail key it became a Win 10 Digital entitlement after one month. This digital entitlement is linked to your email address if you logon using your Microsoft account (the email address you gave Microsoft to register for an upgrade) and according to the changes to activation, you can install Win 10 on this PC as many times as you like.
Three things actually, you don't even need to know the key anymore. when you use the installer and get to stage where it asks for a key, click "Skip" or "I don't have a key" and win 10 will fix up activation once installed. It may activate itself or you may have to run troubleshooter above. I don't know for sure as this is all new.