Firewall/NAT

Hi:
I labbed up a m0n0wall (1.311b) in a fairly vanilla configuration: LAN, WAN & DMZ. I tested the firewall rules and all seemed to work well.  Then I took it into the field to deploy and found that the WAN --> DMZ access no longer worked. 

The only change I had made was to switch the WAN port from DHCP to PPPoE. Would this change break my previously-setup rules? In Linux, if you switch a port to PPPoE, the OS designates the port as PPP rather than, say Eth0, which is enough to break existing firewall rules.  Does the same thing happen under the hood/bonnet of m0n0wall?
--

I don't think that Monowall works in the same way.  Within the rules in Mono you always refer to the WAN interface and not a device ID.  My experience of swapping between PPPoE and DHCP was that the rules worked fine.