I was assisting an amazing client in moving some VMs off an older storage array and onto a newer storage platform. They had some VMs that had Physical RDMs (pRDM) attached to the VMs, and we wanted them living as VMDKs on the new SAN.
Traditionally, I have always shutdown the VM, remove the pRDM, re-add with vRDM, and then do the migration, but found an awesome write-up on a few separate ways in doing this.
(Credit of the following content goes to Cormac Hogan of VMware)

VM with Physical (Pass-Thru) RDMs (Powered On – Storage vMotion):

• If I try to change the format to thin or thick, then no Storage vMotion allowed.
• If I chose not to do any conversion, only the pRDM mapping file is moved from the source VMFS datastore to the destination VMFS datastore – the data stays on the original LUN.

VM with Virtual (non Pass-Thru) RDMs (Power On – Storage vMotion):

• On a migrate, if I chose to covert the format in the advanced view, the vRDM is converted to a VMDK on the destination VMFS datastore.
• If I chose not to do any conversion, only the vRDM mapping file is moved from the source VMFS datastore to the destination VMFS datastore – the data stays on the original LUN (same behaviour as pRDM)

VM with Physical (Pass-Thru) RDMs (Powered Off – Cold Migration):

• On a migrate, if I chose to change the format (via the advanced view), the pRDM is converted to a VMDKon the destination VMFS datastore.
• If I chose not to do any conversion, only the pRDM mapping file is moved from the source VMFS datastore to the destination VMFS datastore – the data stays on the original LUN

VM with Virtual (non Pass-Thru) RDMs (Power Off – Cold Migration):

• On a migrate, if I chose to covert the format in the advanced view, the vRDM is converted to a VMDK on the destination VMFS datastore.
• If I chose not to do any conversion, only the vRDM mapping file is moved from the source VMFS datastore to the destination VMFS datastore – the data stays on the original LUN (same behaviour as pRDM).