Unfortunately there isn't a way to change the physical location of the RDM from SAN to SAN via vmware IF you want to keep the Disk as an RDM. All we can do here is change the pointer file location if this is the case.
You can perform a Storage vMotion and migrate the Virtual Mode RDM into a .vmdk disk if this would be acceptable? Just follow the same process as you would for a normal Storage vMotion.
An alternative would be to create a LUN on the new SAN array, present it to the VM, and then perform an OS level copy of the data over to the new drive.
Sorry to be the bearer of bad news!