I think this should work...but have not tested it myself.
- Install SSO 5.1 on the current vCenter server (which I assume is still on 5.1)
- Back up your existing SSO DB - kb.vmware.com/kb/2034928
- Restore the backed up SSO DB to your new instance
- Associate vCenter with the new SSO instance - kb.vmware.com/kb/2033620
- Then go forward with your upgrade
Good luck -
Rick