You can upgrade a vm $vm to hardware version 8 with:
Get-VM $vm | Get-View | ForEach-Object { $_.UpgradeVM("vmx-08") }
The vm has to be powered off.
Blog: https://rvdnieuwendijk.com/ | Twitter: @rvdnieuwendijk | Author of: https://www.packtpub.com/virtualization-and-cloud/learning-powercli-second-edition