Wow, Oracle's response was not what I had hoped for:
I am Arun from Oracle Support.I will be assisting you further on this SR.
I am sorry to say but you would have re-install the complete Cloud control, including all the agents. As the hostname will be deply embeded in numerous files and it will be impossible to change each and every file.
So I would suggest a re-install if you want to change the domain name
Let me know if you need more information
You can do this, but you'll need to treat it as a restore/recovery scenario. Backing Up and Recovering Enterprise Manager --- follow OMS restored on different host procedures.
There's a slight chance that you might be able to "fake it" with the hosts files to get the system started again, but not sure about Windows....