The Agent uses either the last known IP address, DNS name, or NetBIOS name of the ePO server. If you change any one of these, ensure that the Agents have a way to locate the server. The easiest way to do this is to retain the existing DNS record and change it to point to the new IP address of the ePO server. After the Agent is able to successfully connect to the ePO server, it downloads an updated SiteList.xml with the current information.
After you change, go to the core/config page and make sure SQL is connecting fine also.