From all the information you provided, I assume there might be something in the ADAM database which needs to be cleared/reset.
The license key assignment is stored in the ADAM database (rather than in the vCenter Server database) and with the new vCenter Server installation on the same (old) host this may not have been reset. Please take a look at http://kb.vmware.com/kb/1015268 to see whether this KB helps resolving the issue.
André