Short answer--New Motherboard=New License Key if you have an OEM OS version.
The reason is your license key is embedded in your BIOS, and the BIOS is embedded with the MOBO.
That's not really the reason. The reason is because the terms of the EULA dictate the OEM license MUST stay with the "O"riginal "E"quipment, and we as the users, agreed to abide by those terms when we decided to keep using that license with the original equipment. And that makes it
legally binding.
And with home or custom built computers, the key is not embedded in the BIOS. That is only with factory built computers using UEFI BIOS with Windows pre-installed at the factory. But again, it is the terms of the EULA we agreed to abide by that how we can use the license. And the terms of the OEM licenses dictate they cannot, under ANY circumstance, be transferred to a new computer (or upgraded computer using a new motherboard).
Windows 8 is very forgiving
Yes, Windows 8 is forgiving, but just because you can run a Stop sign and not get caught, that does not make it legal. Same thing here. A new motherboard is considered a new computer and therefore requires a new license - regardless if W8 is forgiving or not.
There are two notable exceptions. (1) If the motherboard is being replaced as part of a "repair" action, BUT it must be replaced with an identical board from the same maker, or a replacement recommended by the maker if the original model is no longer available. You cannot "upgrade" to a better board, even as part of a repair. And (2) if the original license it more expensive, full "boxed Retail" license, purchased separately.
So bottom line, if the old license is an OEM/System builders license (and if factory installed, it surely is) then the
right and
legal thing to do is to buy a new license to go with this new "E"quipment - even though should you decide to violate the terms of the License Agreement you previously agreed to abide by, you will probably get away with it.
Those are the facts - its in your EULAs and I'm just the messenger.