Stop! Do NOT create ANOTHER VM on the Mac. Use the very same VM that boots the BootCamp partiton. For it, create a new Virtual Hard Disk. Install Win8.1 there and upgrade, activating the VM on the way. To finish, switch the VM back to use the BootCamp partition instead of the Virtual Hard Disk. Verify the VM is activated. Then delete the Virtual Hard Disk if desired. Was only needed for activation. Explanation: The VM is the computer, so don't change VMs. Changing (only) the HDD in a computer does not de-activate Windows.