OSSEC hosts can be centrally renamed by editing the etc/client-keys file.

However, since the client key also contains the host name the client will use, the keys must be re-extracted using manage-agents and the keys imported into the existing client installs.

This is only a small time-saving over removing and re-adding a client but less prone to human error.

Edit:
This also works for changing OSSEC host IPs. Simply update them in the keys file and restart the agent and client and they will reconnect.