In our lab we have a mix of ESXi 5.5, 6.0, and 6.5. When deploying a new virtual machine with Orchestrator our template was upgraded to a hardware version of 13, which is only compatible with ESXi 6.5. The one I was working with had to be on a specific host for reporting and testing that was on version 6.0 and this was time sensitive so there wasn't much time to upgrade the host which would have been ideal. So instead, I found this helpful snipit from the VMware community on downgrading the virtual hardware version.
Start by shutting down the virtual machine in the vSphere client and remove from inventory, this was something I had to do anyways, because the host it was moving to was not in a cluster. The virtual machine was migrated to a different datastore prior and then did the following.
Browse to the datastore and find the virtual machine’s .vmx file. Here you can either download it to your desktop, or if you're comfortable with the Linux editor 'vi' you can use that. Of course create a backup of the file. I used:
cp /vmfs/volumes/datastore1/vm-test.vmx /vmfs/volumes/datastore1/vm-test.vmx.bak
Open the .vmx file in Notepad or your favorite text editor and find the following line in the configuration file and change the “13” to a “10” or "11"
virtualHW.version = “10” will become virtualHW.version = “9”
Save the file and overwrite your existing .vmx file. If you downloaded the file to your desktop, upload the file to the same location as the virtual machine from before. Then, add the machine to inventory and start it up on the lower version host or cluster.
