To change the Coordinator serving an Agent without manually modifying the Agent Settings dialog, modify the following registry key (either manually or by executing a .reg file on the required machines) :

HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Xoreax\IncrediBuild\BuildService\CoordHost

After this modification it is recommended to restart the Agent's service in the following way:
Open Agent Settings -> Agent -> General. Click on the "Restart" button.