Move vm's from vCenter Server 4.1 to vCenter Server 5.1 environment

We have vCenter Server 4.1 and vCenter Server 5.1 environments
.vcenter server 4.1 has esx4.1i and vcenter server 5 has esx 5 host respectively.
Now we are planning to migrate vm’s from vcenter server 4.1 environment  to vcenter server 5 environment. Could someone
help me what would be the best method to achieve this other than using vm
convertor method.