Oracle Enterprise Manager (EM)

Create EM at a later time
emca -config dbcontrol db -repos create -silent -ORACLE_HOSTNAME devdb1.com -PORT 1521 -SERVICE_NAME PRDB1.devdb1 -SYS_PWD pa55w0rd -SID PRDB1 -DBSNMP_PWD pa55w0rd -SYSMAN_PWD pa55w0rd -DBCONTROL_HTTP_PORT 1158 -AGENT_PORT 3938 -RMI_PORT 5521 -JMS_PORT 5541

Drop EM at a later time
emca -deconfig dbcontrol db -repos drop -silent -ORACLE_HOSTNAME devdb1.com -PORT 1521 -SERVICE_NAME PRDB1.devdb1 -SYS_PWD pa55w0rd -SID PRDB1 -DBSNMP_PWD pa55w0rd -SYSMAN_PWD pa55w0rd -DBCONTROL_HTTP_PORT 1158 -AGENT_PORT 3938 -RMI_PORT 5521 -JMS_PORT 5541

When the database host name or the IP address changes, deconfigure and then reconfigure the Database Console with the repository create command. Run the following command:
emca -deconfig dbcontrol db -repos drop emca -config dbcontrol db -repos create

On 17 Oct 2012, i had issues / errors createing the above, so finally found, I had to apply the patch since for db 10.2.0.5 certificate was out of date:

PATCH:8350262 - CREATE DBCONSOLE CERT WITH 10YEAR VALIDITY

Good read/doc on this topic in Metalink: Enterprise Manager Database Control Configuration - Recovering From Errors Due to CA Expiry on Oracle Database 10.2.0.4 or 10.2.0.5 [ID 1222603.1]