I got this link from ESET tech support.
hxxp://support.eset.com/zap/kb3701/
By doing a "repair" for the ESET Remote Administrator Agent you can change the ERA IP address
Also this link will help in case you want to create a new Policy for ERA Agent to connect to the new ERA Server
hxxp://help.eset.com/era_admin/63/en-US/index.html?admin_pol_planed_for_migration.htm