Thanks!
Of course, what was I thinking: the web configuration will not be available at first!
But the Supermicro board is a designed server-board and the MAC numbers can (I think) be changed and cloned in the BIOS: Would you recommend this if it were possible or is your suggestion the better way? Anyway, I am well and again living in hope and now I just have to wait for the board to arrive...

You won't be able to edit MAC's in the Webconfig. From a terminal (alt+f2 from the console) you can use the nano editor. Alternatively, if you want to get it ready for when ClearOS boots properly, you can boot up the server with any distro Live CD (or the ClearOS installation CD in rescue mode) and edit the file once you've worked out how the partition has been mounted. Then restart the server into ClearOS.

You may well find the MAC numbers on the motherboard box or on the motherboard itself near the RJ45 sockets. If not, the BIOS might give them or a Live CD. You can't normally change them.

I was hoping that something like this would do: I assume that the system would boot up initially and then I could edit the MACs in the clearos settings? I assume that if I first go into the new BIOS I could copy the new MAC numbers? Or is it another option to take my old MAC numbers (I have them written down) and change the MACs on the new board? The board has two on-board NICs...

If you just reuse the old hard disk your system should boot up, but it is worth making sure the BIOS is set similarly to how it was before, so it is not a dream. It is also not 100%. If everything else is the same the main issue will be your NIC's which will change ethX assignment so you may have to reconfigure your interfaces. It will only be an issue if you use different (including on-board) NIC's. Alternatively (and what I'd do), you can go into /etc/udev/rules.d/70-persistent-net.rules and edit the rules to the correct MAC matched the ethX interface. Then delete the other entries. Rebooting should then give you back the interfaces as you used to know them.