To upgrade to vSphere 4.1, upgrade hosts with service consoles to ESX 4.1 (which also has a service console). Upgrade hosts without service consoles to ESXi 4.1 (which does not have a service console). You cannot use the upgrade tools to convert ESX hosts to ESXi hosts, or the reverse.

VMware provides the following tools for upgrading ESX/ESXi hosts:

vCenter Update Manager

Robust software for upgrading, updating, and patching clustered hosts, virtual machines, and guest operating systems. Orchestrates host and virtual machine upgrades. If your site uses vCenter Server, VMware recommends that you use vCenter Update Manager. See the VMware vCenter Update Manager Installation and Adminstration Guide accessible on the VMware vCenter Update Manager Documentation page: http://www.configuresoft.net/support/pubs/vum_pubs.html

vihostupdate

Command-line utility for ESX and ESXi. This utility requires the vSphere CLI.

esxupdate

Command-line utility for ESX only.