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If you have a need to change the passwords for the sysman or dbsnmp users you will also need to edit configuration files used by Enterprise Manager. These files are $ORACLE_HOME/<hostname>.<instance_name>/sysman/config/emomes.properties and $ORACLE_HOME/<hostname>.<instance_name>/sysman/emd/targets.xml
The passwords may appear as encrypted. You can replace with clear text if you change the encrypted directives to FALSE.