It probably won't be quite that easy. All Soekris 45xx and 48xx use the sis driver and your interfaces will show up as sis0, sis1, .... The 55xx us a VIA nic (
http://soekris.com/products/net5501.html) so they will probably use the vr driver, but I'm guessing there. I *think* you will just be able to re assign ports via the serial console on the first boot and all the rest of your config will just work once the WAN and LAN are properly named per the new driver.
As always, just back up your config and try it