Hi,
if you shutdown and start the VM (or reboot but only when done from the web UI/CLI, not from within the guest), pending changes will be applied during the start. And some parameters can be hot-plugged/changed live.As my experience you have to enable the agent WHILE the vm is off; if you enable it while running the "Enabled" label is red even if you stop and start the VM. I had the same issue and stopping the VM and disabling saving and enabling again the parameter it worked.
As I saw if you modify ANY parameter while the VM is active doesn't work even if you stop and start it. (using the "free" version)