Hmmm

I installed the new LAN card and logged into the system through the WAN interface. The replacement card shows up as a new ethernet interface but it says "no link" on the IP Settings even though I've configured it for a static IP. I booted the machine off a Linux Live DVD and the card's ok.

EDIT : despite the fact that the dashboard says "no link", it's working!

Much simpler than I'd expected!

Thanks for your help, even though in the end I probably didn't need to ask the question in the first place.

This depends on how adventurous you feel. The direct way is to add the network card and reboot. Log on to the console and note the IP address of the failed interface. If you want to have the same settings on the new interface, you have to either delete this interface or change its network setting to something in a different subnet. Then configure your new interface as LAN with the old IP address of the old interface. You can't get to the DHCP server here. If you can get to the Webconfig via the WAN IP then do that and go and delete the old DHCP server and configure the new one. Then plug into the new LAN and it should work. If you can't do that, plug you PC into the new NIC and if you don't get a connection/IP address, you'll need to manually configure the PC with an IP address on the same LAN as the new NIC. You'll then be able to connect to the Webconfig and configure the DHCP servers correctly.

If you feel adventurous, once you've powered up ClearOS with the new NIC, note the LAN interface name and the new card name. Then drop to the command line in the console and edit the file /etc/udev/rules.d/70-persistent-net.rules with either vi (yuck) or nano and swap over the "NAME" of your working and non-working interfaces. Save the file then reboot - use the command "reboot" from the command line. You should then be up and running again.

Note Windows may well detect a new network either way as the LAN interface has a new MAC address.