If you have the full version (not oem) of windows you can reinstall on different hardware without a problem. It is only the oem version that requires a new license.
EDIT: SORRY! I made a mistake Actually the oem windows 8 license is different. I do believe it allows you to transfer it to different hardware, provided the original is deactivated. More info: http://personaluselicense.windows.com/en-US/default.aspx (this does not apply to manufacturer installed copies on windows 8 oem)
Windows 8 does not follow the same licensing as Windows 7.
It used to be (win 7) that you could not change the MOBO (only piece) with an OEM license you would have to buy a new one. With Windows 8 you can transfer the license to an entirely new pc as long as it is only active on one PC at a time. The new Win 8 licensing is much like the Win 7 RETAIL licensing.
I should have explained my situation in full. On this PC I took advantage of the cheap offer to upgrade from vista to windows 8. I still have my vista OEM disk and code. What are my chances of getting Microsoft to allow me to swap motherboards?
I believe the rules stay the same. That you may transfer and replace parts still. The issue you might run into... is you'd have to follow the same upgrade path. Vista etc. I don't know if you would run into issues there. Check out this forum.