It looks like the hardware version can be downgraded from 10 to 9. Below is a procedure from: How to Downgrade VMware ESXi 5.5 Virtual Machine 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 virtualHW.version = “9″ 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. - See more at: http://blog.waldrondigital.com/2014/01/21/how-to-downgrade-vmware-esxi-5-5-virtual-machine-hardware-version-to-9-from-10/#sthash.Dqrke6JY.dpuf