Problem to add host VCENTER

Hi guys.

I have worked with VCENTER 5.1 with  ESxi 4.1 and ESXi5.1 a two week ago I have started migration hosts from old VCENTER to new VCENTER 5.1, when start process add host the process fail happens the message "Can not  assing the license key to the selected assets" . I have used the same  the license key.  When I put  another key works very well, but I need use the same key. There is other problem, before add that host I remove the host of  the old VCENTER and when I trie add host on new VCENTER happens the message that HOST is already being managed by old VCENTER ;

Someone could help me ?