I am assuming an environment of VMware VShphere 5.5 where you have your VMs running on a version 8 virtual hardware.
If you are creating new virtual machines on VSphere 5.5, it will be creating the VM with the virtual hardware 8. VShpere 5.5 has support up to version 10 virtual hardware. But keep in mind if you are converting your existing VM hardware to version 10 you will only be able to manage it from the VCenter web console because the VCenter appliance will only support up to version 9.
So if you are converting your virtual hardware versions to 10 you can’t revert it back. You can do the virtual hardware upgrade from Vcenter appliance but it will convert straight to 10.
So if you still want to manage your VMs from the VCenter Appliance the best is to convert it to version 9. But how?
You can do this from the VCenter web console. Follow the below steps to do it.
1) Login to the VCenter web console.
2) Then select the VM that you need to perform virtual hardware upgrade and right click on it and then click on “Edit Settings”
3) Then click on the Upgrade tab to view all the options.
4) Now select the Check box “Schedule VM Compatibility Upgrade”
5) On the “Compatible With’ drop down box select ” ESX 5.1 and later” – This will do the hardware upgrade to version 9.
6) Now select “Only apply after normal guest OS shutdown”
Once you restart the VM next time the Upgrade will happen and you will your VM hardware version changed to 9.
PS : It is also a best practice to take a snapshot or do a backup of the VM before you do the Virtual hardware upgrade.