From LedHed's Wiki
Smoothwall doesn't allow you to change the 'Green' interface after the initial installation. This can be a problem if your Green NIC fails and needs to be replaced.
The solution is to edit /var/smoothwall/ethernet/settings
GREEN_DRIVER=eepro100
Upon changing the Green Interface driver reboot and the new NIC should be detected.
To check if the changes took place after a reboot. Use ifconfig to make sure the MAC address corresponds to the new NIC. You can also:
ping 127.0.0.1