I have went through the upgrade process of vCenter 5.0 to vCenter 5.1 which was protected by vCenter heartbeat(Please note same process apply for upgrade to 5.5 as well). The process end up being more of uninstalling and re-installing vCenter heartbeat rather than upgrade to avoid the undesired behavior of SSO lockdown that can be caused by the upgrade process of vCenter Hearbeat when SSO is hosted on the same VM as vCenter which was the case in my scenario. Let start by giving a summary of the upgrade process before going into a details:

a.  Uninstall vCenter Heartbeat 6.5 if installed
b.  Setup Single Sign On.
c.  Upgrade vCenter Server from to 5.0 to 5.1
d.  Upgrade Update Manager from 5.0 to 5.1
f. Install vCenter Heartbeat 6.6 (UAT & Prod).

below is the process  changes involved when upgrading vCenter that is protected by vCenter Heartbeat in more details:

1-      Preparation:

• Make sure you have adequate backup. Maybe Image backup of your vCenter VMs?
• Make sure to have license keys on hand.
• Make sure All Windows Updates is disabled.
• Write down all the IPs configurations.
• Prepare a service account to use for heartbeat with local admin and vcenter admin privillage
• Make sure to have a local administrator account on the machine that you know the password off.