VMware KB: Re-pointing and re-registering VMware vCenter Server 5.1 / 5.5 and components
Note that you will need to point the Inventory Service as well, and then also reregister vCenter to the Inventory Service.
If the Web Client fails to repoint to the new SSO instance it's no big deal as it's a complete binary package and a simple uninstall and reinstall pointing to the correct lookupservice URL will "solve" the issue.