I experienced this same problem after moving my ESXi 5.0 U1a host back to a standard switch for some testing. Once I moved my host back, in this case, to a 1000v it moved back fine but I had to recreate my vMotion VMkernel port and got the message "A vmkernel nic for the connection point already exists". I sshed into the host and checked to see if the VMK1 existed and it did. Command: esxcli network ip interface list
I issued the command to delete it. Command: esxcli network ip interface remove --interface-name=vmk1
Once I deleted vmk1 I was able to recreate it via the client through vCenter with no issues.
Hope this helps.