To prevent that from happening, AMU intentionally disallows modification of these settings. It will, however, report status of these settings.
When you need to change Serve DHCP and Provide NAT settings, use AirPort Admin Utility to do so on an individual basis.
Note: AirPort Management Utility 1.0 may be found on the software CD included with the AirPort Extreme Base Station (PoE/UL 2043).