Thanks to the export/import function now available in vSphere 5.5 and 6.x, we can now move the full distributed vswitch configuration to the new vCenter:
- In the old vCenter, right-click the switch object, click "Export configuration" and choose the default "Distributed switch and all port groups"
- Add the hosts to the new vCenter
- In the new vCenter, right-click the datacenter object, click "Import distributed switch" in the "Distributed switch" sub-menu.
- Select your saved configuration file, and tick the "Preserve original distributed switch and port group identifiers" box (which is not default!)