Agent configuration in databases Grid Control

Delete the targets from OMS including the old agent registration;
Log into Grid
-> Targets
-> All Targets
-> Search for targets of type Agent
-> Select radio button of one of the Agents of status
-> Remove
You will get an error screen that it is monitoring targets and cannot be removed.
-> Follow the "Click here for the list of targets running on the host".
-> Select All
-> Remove

The non-clustered targets will be removed after you click OK. The clustered targets have to be removed one at a time.

-> Select one of the remaining clustered targets (if any on that host).
-> Remove

You get a warning about the agent cannot be contacted.

Check the "Ignore the status of the agent(s)" check box.

Press YES. Repeat as necessary.

Only the two competing agents should still be listed on the node.

Select the one with status and Remove.
This may give you an error that it's still monitoring targets.
Press OK
Repeat steps h - p as necessary until it finally removes all databases/listeners/asm.

When nothing but the two agents appears on the list but the agent still won't delete, follow the link to the old agent and remove the host from the list of monitored targets on the agent status page.

Repeat steps b - f and the agent should finally go away.

Finish removing all targets before proceeding to rediscovery.
Rediscover targets on the nodes via the new agent.
Log into Grid
-> Targets
-> Databases
-> Add
Click the flashlight
Select the host you just cleaned out in step 1.
-> Select
-> Continue
If prompted whether to discover targets only on this node or on all hosts in the cluster, select all hosts in the cluster and continue.

Wait for discovery to complete (this can take a while especially if the node is clustered).
The list of discovered targets will be displayed. Any targets with a pink wrench will need you to specify the connection password. If you don't know or can't find the current value, just reset the password and then enter the new password in the configuration screen.

Even after discovery completes, you may still need to find and configure the dbsnmp passwords for some of the databases.