VMware 6, changing VM MAC address to MAC address of physical machine

I did a P2V on a server and the software installed on the server is keyed to the MAC address of the physical machine.  So, thinking myself clever, I decided the change the MAC address of the VM to that of the physical machine.  Alas, I cannot!

I'm using the client to access my vcenter and then I edit the settings for the NIC and click the manual radio button for the MAC.  When I do, it repopulates with 00:50:56.  So, I paste in my MAC address from my physical machine and try to apply that and I get the message "The MAC address entered is invalid.  Enter a different MAC address of choose a different assignment method."

I did read that I can do this by editing the VMX file but I wanted to post here first and get guidance.

Can I set the V-NIC to the same MAC address of an old physical machine that is never coming back again?

IF yes, how?

Thanks

Cliff