check this blog and discussion,
http://blogs.vmware.com/vsphere/2012/02/migrating-rdms-and-a-question-for-rdm-users.html
same process:
The ability to migrate the data from one RDM LUN to another would be extremely valuable. We are in a constant state of data migration, which takes immense planning preparation, and scripted execution. It would free us from a very arduous process.
1 – initiate data copy between arrays
2 – shut down VMs
3 – resync data
4 – unmap rdms from virtual machines
5 – unpresent original luns from hosts
6 – present new luns to hosts
7 – map rdms to new luns
8 – bring vms back online
This was good discussion in EMC forum,