For those of you who usually use the "legacy" vSphere C# Client to perform virtual machine virtual hardware upgrade (also known as Virtual Machine Compatibility) should know that the default behavior is to automatically upgrade to the latest supported version. This is usually not an issue, however with vSphere 5.5 if you do perform this upgrade, one caveat to be aware of that you will NOT be able to edit the virtual machine configurations using the vSphere C# Client afterwards. A confirmation dialog is even presented to warn the user before performing this operation and that the virtual machine can only be manage through the vSphere Web Client.
Note: Even though the virtual machine settings can not be managed/configured using the vSphere C# Client, you can still use the various vSphere API/CLIs to manage the virtual machine and those are fully supported.
I had noticed a couple of comments on Twitter the other day and even at VMworld Barcelona that this was not ideal that the vSphere C# Client automatically upgraded to the latest version. I know there are some folks that would have liked to upgrade to a specific version of virtual hardware. Luckily, you can easily do so by using the vSphere API/CLI such as PowerCLI for example if you have paid vSphere license.
You can use the Set-VM cmdlet and specify the -Version property, here is the syntax for the command:
Set-Vm -VM (Get-VM -Name [VM-NAME]) -Version v[HW-VERSION]
Here is a screenshot of upgrading a VM called "Duncan" from vHW8 to vHW9:
Now this is great for customers who have a vSphere license that allows for both read/write access to the APIs which PowerCLI and other CLIs leverage. For customers using Free ESXi or just want a quick and simple way of upgrading to a specific virtual hardware version, you can leverage vim-cmd utility which is found in the ESXi Shell.
You can use the following command to upgrade to a specific virtual hardware version (you will need to specify the VM-ID by using vim-cmd vmsvc/getallvms):
vim-cmd vmsvc/upgrade [VM-ID] vmx-[HW-VERSION]
Here is a screenshot of upgrading a VM called "Cormac" from vHW7 to vHW9: