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, Vinod