Thanks for your reply, do we need to re-install VC, I would rather prefer to run upgrade of vCenter 5,5 over my existing vCenter 5.1. Secondly, what if take SSO DB backup, uninstall it from its dedicated virtual machine. Re-install SSO 5.1 on my vCenter pointing to backup SSO DB and then run SSO 5.5 setup so it would detect earlier version and proceed for SSO 5.5 upgrade.
BR,