Hi Sam
1. For Client copy in both system Support pack level should be same. as already mention in the SAP link .
Copying Clients Between Systems (SAP Library - BC ? Client Copy and Transport (BC-CTS-CCO))
2. If the SPS level are different,you have to perform system copy (Heterogeneous system copy method) since the OS are different and then remove the unwanted clients.
Regards
Sriram