I had a circumstance where the hostname changed on a box, and although there was only one agent on the box, grid control was confused about the original agent, and it's stats was "Agent Unreachable". Oddly, it showed that the last upload time was the same as the reachable agent. The solution is quite simple: perform the following steps as indicated in metalink doc 735062.1:
"After any change on the host configuration, apply the following steps.
In Grid Control navigate to:
-> Refresh Host Configuration link
-> Select the host in the "Available Host" menu
-> Click on "Refresh Hosts" button.
Make sure that the submitted job finishes correctly and check the host home page to confirm that the changes have been propagated to the repository"
It worked perfectly for me on GC 10.2.0.5. The old agent is gone from GC now. Yeah!
Tips and wisdom from 15 years of DBA experience