It's always advisable to do a fresh install when installing a new motherboard. If you try to boot with the old installation, in many cases it will BSOD before you get to log in.
As for not being able to transfer the product key from one motherboard to the next, Rationale is correct if we are talking about Windows 7 and older. However since MicroSoft did away with the Retail version of their OSes with Windows 8, that restriction only applies to System Builder machines (HP, Dell, etc), you should have no trouble using Windows 8 on another motherboard. You just can't use it on more than one system at a time, just like the older Retail versions.