When you upgrade to vCenter Server, no downtime is required for the ESX/ESXi hosts that vCenter Server is managing. Nor is downtime required for the virtual machines that are running on the hosts. Downtime is required for vCenter Server.

Expect downtime for vCenter Server as follows:

VMware estimates that the upgrade requires vCenter Server to be out of production for 10 minutes, plus 15 to 20 minutes for the database schema upgrade, depending on the size of the database. This estimate does not include host reconnection after the upgrade.

If the machine does not have Microsoft .NET Framework already installed, a reboot will be required after the upgrade to vCenter Server.

VMware Distributed Resource Scheduler does not work while the upgrade is in progress. VMware HA does work during the upgrade.