I had to migrate an OEM 12c CC (OEM Cloud Control 12.1.0.5) to a new server. The existing OEM had all 3 tiers (web tier, OMS and OMR database) running on the same Solaris server. The new server is in a new location with new hostname and IP's. I found Oracle support document on how to change IP address for OEM OMS server, but I couldn't find any good document of how to migrate OEM/OMS to new server with different hostname. I tested out the procedure to change hostname for an OMS server. I would like to share to steps I tested. Hope this will help other with the same situation.
The following is a summary of the steps to change hostname for OMS server. Please find the full example at http://expertdba.weebly.com/blog/change-hostname-for-oem-cloud-control-12c-oms-server
Step 1. Take a backup of OMS configuration from the OMS host
Command: <OMS_HOME>/bin/emctl exportconfig oms -dir <backup dir>
Step 2. Stop OMS
Command: <OMS_HOME>/bin/emctl stop oms -all
Step 3. Change OMS server hostname by restoring backup of OMS configuration
Command: <OMS_HOME>/bin/omsca recover -NOSTART -BACKUP_FILE <OMS_EXPORTCONFIG_DUMPFILE> -AS_HOST <NEW_HOSTNAME> -EM_INSTANCE_HOST <NEW_HOSTNAME> -AS -MS
Step 4 Start OMS
Command: <OMS_HOME>/bin/emctl start oms
Step 5 Check OMS Status
Command: <OMS_HOME>/bin/emctl status oms -details
Step 6 Redirect OEM agent
Follow "Enterprise Manager Cloud Control Advanced Installation and Configuration Guide" Appendix C "Redirecting Oracle Management Agent to Another Oracle Management Service" to redirect OEM agent