First, verify that the agent is responding to requests, by navigating to the new IP/host with your browser:
https://my.new_server.lan:3872/emd/main/Then log on to the OEM repository DB as sysman. The table MGMT_TARGETS holds all the juicy agent details. Just update the column EMD_URL with the new value, and everything is fine.
update mgmt_targets set emd_url=replace(emd_url,'my.server.lan','my.new_server.lan') where target_name like '%my.server.lan%';Verify your changes in the view MGMT$TARGET.
Finally the agent should be available in Setup - Agents at once. I then resync'd the agent from EMS, and secured the agent. All targets were up and running without issues then.