I was able to find the following document which lists all of the default ports used by Enterprise Manager (10g).
Subject: Overview of Default Ports Used by EM 10g Grid Control, DB Control and AS Control
Doc ID: Note:235298.1
Search for "SQL*Net Listener for the 10g DB" and it will list the modifications required in order to change the listener port. The steps in this section assume you need to change the listener port as well. Since the listener was already created for me here are the steps I had to follow:
1. Stop DB Control by executing the command emctl stop dbconsole
2. Modify the agent to reflect the new port number. Edit $ORACLE_HOME/<hostname_sid>/sysman/emd/targets.xml
The instructions say you only need to modify the listener port value. However, I noticed that there are two entries in this file. One is for the database and another for the listener, each pointing to the default port of 1521. I'm not sure if I needed to change the database port but since the DBA wanted to keep both environments separate I changed it to be safe. I didn't test this procedure without making that change.
Here is a copy of the file (of course username and password values were modified. ;).
<Targets AGENT_SEED="39661000">
<Target TYPE="oracle_emd" NAME="myserver.mydomain:1830"/>
<Target TYPE="host" NAME="myserver.mydomain" DISPLAY_NAME="myserver.mydomain" VERSION="1.0"/>
<Target TYPE="oracle_database" NAME="mydb10g" VERSION="1.0">
<Property NAME="MachineName" VALUE="myserver"/>
<Property NAME="Port" VALUE="1522"/>
<Property NAME="SID" VALUE="mydb10g"/>
<Property NAME="OracleHome" VALUE="D:\oracle\ora10g"/>
<Property NAME="UserName" VALUE="1993cs3901f8d5af7" ENCRYPTED="TRUE"/>
<Property NAME="password" VALUE="8c35Da9iks4e07bab7b6c5906a837f4" ENCRYPTED="TRUE"/>
</Target>
<Target TYPE="oracle_listener" NAME="LISTENER_myserver" VERSION="1.0">
<Property NAME="Machine" VALUE="myserver"/>
<Property NAME="LsnrName" VALUE="LISTENER10g"/>
<Property NAME="Port" VALUE="1522"/>
<Property NAME="OracleHome" VALUE="D:\oracle\ora10g"/>
<Property NAME="ListenerOraDir" VALUE="D:\oracle\ora10g\network\admin"/>
</Target>
</Targets
3. Modify $ORACLE_HOME/<hostname_sid>/sysman/config/emoms.properties to reflect the new port. Two entries have to be modified here as well. emdRepPort and emdRepConnectDescriptor should reflect the new port number.
Here are the modified entries with 1522 replacing the standard port 1521.
oracle.sysman.eml.mntr.emdRepPort=1522
oracle.sysman.eml.mntr.emdRepConnectDescriptor=(DESCRIPTION\=(ADDRESS_LIST\=(ADDRESS\=(PROTOCOL\=TCP)(HOST\=myserver)(PORT\=1522)))(CONNECT_DATA\=(SERVICE_NAME\=mydb10g)))
4. Last but not least start up database console with emctl start dbconsole and verify that you can login successfully.