Segundo a Microsoft, uma chave do Windows 8 fornecida a partir do OEM não é transferível:
Q: My customer bought a new PC and wants to move the OEM software from the old PC to the new one. Can't users do whatever they want with their software?
A: No, the OEM software is licensed with the computer system on which it was originally installed and is tied to that original machine. OEM licenses are single-use licenses that cannot be installed on more than one computer system, even if the original machine is no longer in use. The End User Software License Terms, which the end user must accept before using the software, state that the license may not be shared, transferred to, or used concurrently on different computers
Reference
Esse é o legalismo por trás disso. Na medida em que tecnicamente, você pode fazê-lo, as licenças OEM do Windows 8 parecem estar ligadas a hashes de hardware que incluem a placa-mãe:
manufacturers will be required to write a unique Windows product key -- which is associated with the hardware hash -- into the system's BIOS
Portanto, não, parece que, mesmo com a chave OEM original e a mídia de instalação, a chave está vinculada ao hardware em que está instalada.