vSphere 5.5 Update 2 has just released and among the various bug fixes, one that stands out the most to me and I am sure everyone will be quite happy about (including myself) is the ability to now edit a Virtual Hardware 10 Virtual Machine using the legacy vSphere C# Client. Previously, if you tried to edit a Virtual Machine running the latest Virtual Hardware (version 10), you would get a warning message prompting you to use the vSphere Web Client and the operation would be blocked.
The feedback has been loud and clear from customers/partners and I am glad to see that VMware has re-instated this functionality in the latest vSphere 5.5 Update 2 C# Client. You will still be prompted with a dialog noting that only Virtual Hardware Version 8 features will be editable using the vSphere C# Client and that all newer Virtual Hardware features, you should still leverage the vSphere Web Client and/or vSphere API. This will at least allow you to edit basic functionality of a VM when vCenter Server is unavailable or if you are not running vCenter Server but wish to use the new Virtual Hardware version.
Here is the direct download link for the vSphere 5.5 Update 2 C# Client: http://vsphereclient.vmware.com/vsphereclient/1/9/9/3/0/7/2/VMware-viclient-all-5.5.0-1993072.exe
Note: You do not need to install vSphere 5.5 Update 2 to be able to use this new functionality, you just need to upgrade your vSphere C# Client to the vSphere 5.5 Update 2 release and you will be able to connect to previous versions of vSphere 5.5 (vCenter Server & ESXi).