Don't forget you will need to import your Agent-Server communication keys from your old ePO server to your new one
Or load new keys into the agent. The KB entry below is for version 4 of the agent so i'm not sure it it works for 3.6
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Use this task to change the server with which a client communicates.
The security keys must be located in the same folder as the site list.
frminst.exe /siteinfo=<full path of target servers sitelist.xml file>
If I'm right CMA 3.6 didn't use communication keys (it was not yet designed for) so if all computers are using this version then he shouldn't need to export security keys.
In fact it's easy to test, he just needs to change the value at the registry key and launch a cmdagent /p command to see if it reports to new epo server.
Going back to the EOL status, please take a look at support article: KB51573 - Supported environments for Common Management Agent and McAfee Agent 4.x
This defines the scope of function of the agents, including CMA 3.6.x
Note at the least that your agent version just won't work on recent OS such as Windows 7, Windows 8, Windows Server 2008 R2 and Windows Server 2012.
Please also take a look at: KB51111 - Supported environments for VirusScan Enterprise on Microsoft Windows
This version of the agent does not meet the minimum requirement for any currently supported version of VirusScan enterprise either.
I appreciate the operational difficulties in rebuilding your image but you really do need to plan to move to a more recent, supported version.
one thing you can try is to create an Alias record for the new epo server using the old servers ip address. when the clients attempt to lookup the old server using DNS they will be forwarded to the new one. Another issue you may encounter as Tristan stated is the security keys, if you do not have the old servers agent keys on the new server they will not be able to communicate with it.