If the OP has a OEM Windows license, then it is "likely" that he would need a new Windows license if/when he installs a different motherboard, since OEM license keys are tied to the mobo.
"You can just move hard drive to new motherboard and re enter the key to activate the OS"
Are you meaning re use the drive, after formatting it, and installing Windows?
Because just booting the pc with a completely different mobo/cpu with the old install of Windows leads to much badness.
It is possible to get Windows to allow his current license to be declared a valid copy, even after a hardware change of this nature, sometimes needing a call to Microsoft, and a little begging. Counting on that happening as a garanteed thing is not such a good idea.