With VMware ESXi 5.5 and vSphere Hypervisor 5.5 it is possible to upgrade the Virtual Machine to hardware version 10. The vSphere Client cannot edit settings of VMs that have hardware version 10. When you try to edit a hardware version 10 in the vSphere 5.5 client the following warning appear:
You cannot use the vSphere Client to edit the settings of virtual machines of version 10 or higher…….
To edit settings of a version 10 VM you need the VMware vSphere Web Client! vSphere Hypervisor 5.5 does not come with the VMware vSphere Web Client, so don’t use version 10 with the vSphere Hypervisor 5.5 else you are unable to edit the settings of a VM! Hardware Version 10 is only compatible with ESXi 5.5 and later.
--> Please see the rest on the blog, but be careful with Virtual Hardware 10: http://www.ivobeerens.nl/2013/10/01/watch-out-with-hardware-version-10-in-vmware-esxi-5-5/