If your deployment uses load balancers to manage multiple Connection Server instances, an upgrade of the Connection Server infrastructure can be performed with zero down time.
After you have performed a fresh install or upgraded all Connection Server instances to the new version of VMware Horizon 8, you cannot downgrade the Connection Server instances to a version earlier than Horizon 7 version 7.2 because the keys used to protect LDAP data have changed.
To keep the possibility of downgrading Connection Server instances while planning an upgrade to the new version of VMware Horizon 8, you must back up the Connection Server instances before starting the upgrade. If you need to downgrade the Connection Server instances, you must downgrade all Connection Server instances and then apply the backup to the last Connection Server that is downgraded.
When upgrading from a version earlier than Horizon 7 version 7.8, some user authentication settings will change. How these user authentication settings affect the user experience depends upon the client. See the Horizon Client documentation at https://docs.vmware.com/en/VMware-Horizon/index.html. You must understand the usability and security implications of the user authentication settings before changing them. See, "Security-Related Server Settings for User Authentication" in the Horizon Security document.
If you have a perpetual or term license, after upgrading a Connection Server in a pod to the new version of VMware Horizon 8, you cannot start a desktop from this Connection Server but you can start a desktop from other Connection Servers in the pod. To start a desktop from the upgraded Connection Server, start Horizon Console on this Connection Server and enter a license key for the new version. Then you can start desktops from all Connection Servers in the pod.
After upgrading a Connection Server instance, perform a hard reload of the Horizon Console browser, so that it picks the latest HTML5 UI source code from the Connection Server.