How to Downgrade VMware ESXi 5.5 VM Hardware Version to 9 from 10

Shut down the virtual machine in the vSphere client.

Remove your virtual machine from the ESXi inventory.

Browse your Datastore and find the virtual machine’s .vmx file. Download it to your desktop.

Open the .vmx file in Notepad or your favorite text editor.

Find the following line (usually line 3) in your configuration and change the “10” to a “9”
virtualHW.version = “10” will become [b]virtualHW.version = “9”[/b]

Save the file and overwrite your existing .vmx file.

Upload the file to the same location your downloaded the file from in step 3.

Add the machine to inventory and start it up.
http://blog.waldrondigital.com/2014/01/21/how-to-downgrade-vmware-esxi-5-5-virtual-machine-hardware-version-to-9-from-10/