One of my friend told me that they used to have Oracle Enterprise Manager Grid Control 10g and deployed its agents to some database servers but now they removed Grid Control and want to use Oracle Enterprise Manager Database Console. Usually people ask the opposite question 
Anyway, in this blog post, I’ll describe how to manually remove Enterprise Manager Grid/Cloud Control Agent and then enable Enterprise Manager Database Control on the target system. First we need to stop the agent:
$AGENT_HOME/bin/emctl stop agent
If your Cloud Control is up, we need to remove agent. Login as oracle user and run the following commands:
emcli login -username=SYSMAN emcli sync emcli delete_target -name="targetsystem.testdomain.com:1836" \ -type="oracle_emd" -delete_monitored_targets
If you have Grid Control 10g/11g, then login to the repository database as SYSMAN and execute the following:
exec mgmt_admin.cleanup_agent('targetsystem.testdomain.com:3872');
Now we’re ready to uninstall the agent software. Run the installer:
export ORACLE_HOME=export ORACLE_HOME=/u01/app/agent/core/12.1.0.2.0 $ORACLE_HOME/oui/bin/runInstaller -deinstall -removeallfiles
Click “deinstall products..” button to see Inventory (installed Oracle products on the server). For Grid Control Agents, all you need is to select agent home and click “Remove”. For Cloud Control Agents, you need to first select and remove plug-in homes (OraHome1..5), then select and remove sbin12cX home, and lastly select and remove agent home.
After we uninstall the agent, we’re ready to reconfigure Enterprise Manager Database Control. All you need is to run the following command. It will ask us to enter some information such as the Database SID, listener port number, passwords of SYS, DBSNMP and SYSMAN users:
emca -config dbcontrol db -repos recreate
After a few minutes, you’ll see that Database Control is up.