You must be refering to an OEM version of Windows 8.
With Windows 7 and earlier an OEM Version was tied to the MB first installed on - If the MB did not die, you technically could not replace the MB.
HOWEVER; with Windows 8 OEM version:
Seems uSoft made a change in OEM windows 8. Under Personal Use License.
"From the Horses Mouth"
Q. What happens if I build a PC with a Personal Use License, then later decide to sell it?
A. You may transfer the software to another computer that belongs to you....... (providing you remove it from the computer)
LOL - Selling a Computer and Reuse the OEM version is way more than just replacing the MB!!!! Glad to see they did this.
Replacing the MB is a NON-issue with Other versions of Window xp, 7, or 8
Bottom Line, Upgrading the MB is a Non-issue with ANY of the Windows 8 versions.
YOU do NOT have to pay for a 2nd lic when replacing MB, even with OEM version!!!!