As others have mentioned, Storage vMotion can move VMDKs between datastores on the same or different array, however this does not work with Physical Mode RDMs (which is most likely the type of RDM used).  You will need to either create a new RDM on the second array and migrate the data within the VM to the new disk, or migrate the LUN itself to the new array, and then remap it to the host.  In either case you will have some sort of downtime for the VM that is using the RDM.  If you migrate within the VM, you just attach the new RDM to the VM, partition/format, then use whatever tools are available within the Guest OS/Filesystem to move the data and redirect the application.  If you have SANCopy, you could push the LUN from the source array to the target array, then take a brief outage of the VM, during which you would perform an updated copy with SANCopy to get any changes, then remove the old RDM from the VM, add the new RDM to the VM and start up the VM.
how do I migrate a rdm lun on a vm from a cx380 to a cx340?
migrating rdm lun to different lun