There used to be a Fling from VMware Labs to assisst you in doing so. Currently unavailable unfortunately.
So there currently is no way of migrating all your data cleanly.
You'd have to build up the VCSA with the external PSC as a seperate entity and then just add the hosts from the old vcenter.
Caveats:
- If you use dvSwitch you'll need to migrate all VM Networking after the switch (of course you'll need to import the old dvSwitch with ports and all other ids staying the same)
- you'll need to recreate your entire tree (permissions, folders, storage clusters, etc.)
- you'll need to re-add all your vms to the correct folder in your vms&templates view because they all get moved to the same folder when you add a host with running vms
this approach is not fully supported by vmware but we've done it lots of times with our customers. if you need further information I could point you to some good blog posts about this.
there are ways to script a lot of the recreating of the objects in the new vcenter but you'll have to figure out if the effort is worth it (I don't know how big your environment is).
