Found this article tonight that saved my behind, http://www.elasticsky.de/en/2012/11/vsphere-5-1-static-mac/#comment-411
Basically the same as someone posted above. I booted the machine, and choose "I moved it" and received the invalid mac error. So I made a backup of the VMX file, changed the mac to automatic, booted the machine with the new automatic mac and upgraded the tools and hardware etc. Then I shut down the VM, edited the VMX file (made backup again) changed the mac type to "generated" and put in my org mac address at the bottom of the file for ethernet0.address, and then booted the machine, leaving the network card type on automatic in the edit vm settings (while the machine was off)
I then booted the VM and it's working fine. The network card setting, when you edit the running vm, shows my old mac address, and is set to automatic. I did not have to modify the uuid or any other settings. Hope this helps.
And for those who say "just get a new license key" That may not always be possible with out major expense. For example in my case, this is old software no longer under maint. It's scheduled to be upgraded/migrated next year, the money is just not in the budget this year to do so.