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 25-30 minutes, depending on the size of the database. The database schema upgrade takes approximately 8 minutes of this time. This estimate does not include host reconnection after the upgrade.

If the machine does not have Microsoft .NET Framework 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.