Scenario: A Hyper-V host is managed by System Center 2012 Virtual Machine Manager (VMM) and it is hosting service templates. You want to reinstall the VMM agent on the host.
If you remove the host from VMM it will retain the VM information but all service template information will be lost. This will prevent any updates to service templates as well as scale out actions.
Logon directly on to the host where you want to reinstall the agent. Remove the agent and reinstall it manually using the latest agent files. On the VMM server, perform an add host action and select re-associate. This will re-register the host in VMM and the service templates will be restored.