When you upgrade Horizon View, you have to upgrade all Connection Servers within the environment. When you just have one, it is clear you will have a downtime. But even when you have n Connection Servers, you have to stop the View Connection Server services of ALL servers at once! This WAS right, but now you can upgrade one Connection Server after the other, without stopping all services of all servers at once. This new procedure you can find in the current upgrade guide here. Take care you download the latest version of the document (ID EN-001476-01) because previous versions – including EN-001476-00 – still want you to shutdown all services.
Dale Carter wrote an short post about the steps to do an upgrade without any downtime.