Install Parallels 8 on Mac and still use old PC?

Is there a legal way to transfer Windows 8 to my new Mac (using Switch to Mac edition) and still use my old PC if I purchase a second Windows 8 upgrade? The reason I ask is my original PC installation of Windows 7 was from a Windows 7 Family Pack (3 licenses). So far, I have only used 1 of the 3 Windows 7 licenses. Can I purchase another Windows 8 upgrade license and change the product key and activate in the virtual Mac instance?

Is this legal and/or possible?

Thanks!