vSphere

Upgrade to ESX 4 (vSphere) on a standalone Host

This article was really helpful

 http://vm-help.com/esx40i/ESXi40_upgrade…

If highly recommend following it if you need to an in place upgrade on a stand alone ESX box.

Unable to downgrade vSphere essentials licenses

The Vmware license portal allows you to downgrade your vSphere license to an equivalent ESX 3 license. However be aware that vSphere essentials licenses CAN NOT be downgraded. Which means if you purchase these licenses you have to use them on vSphere.