On each agent host, stop the agent.

ambari-agent stop

Remove old agent certificates (if any exist).

rm /var/lib/ambari-agent/keys/* 

Using a text editor, edit /etc/ambari-agent/conf/ambari-agent.ini to point to the new host.

[server]

hostname={new.ambari.server.fqdn}

url_port=8440

secured_url_port=8441