if it's an OE version of Windows that you have on, say, a preconfigured Dell machine, than if you change the mobo, it'll deactivate. if it's a system that you built yourself, it should be fine
No way to tell if it will actually deactivate until you do it.
But reactivation is easy. If it doesn't do it online, a phone call to MS will do it.
An OEM install will not work.
As always, back up all important data beforehand, just in case.
Just wandering how I could check if my windows copy is under OEM License?