I'm working on an upgrade project at the moment. It requires me to move guest VMs from a 3.0.1 host onto a host running 4. First I need to move the 3.0.1 host to vCenter 4. I know that ESX 4 cannot be added to vCenter 2.5 according to the compatibility matrix (). This matrix also states that 3.0.1 hosts can very well be added to vCenter 4 (and vSphere 4). However, one of our engineers says he tried and couldn't do it, complaining that it cannot be done. So, before I attempt it myself I would like to ask if anyone actually has done it or my engineer is right and it cannot be done.