No, you can move it
As above states, if you only have it on one pc at a time, you can move the retail copy to your next pc, just have to deactivate it on the old one first
So even though you using everything else the same, swapping motherboard/CPU is seen as a new PC to Microsoft.
Here are instructions on how to do that:
http://www.thewindowsclub.com/deactivate-uninstall-windows-product-key
if you have any problems with the transfer, contact Microsoft and tell them what you are doing.