I was previously with BigPipe using the BigPipe modem (HG659) and when I moved to Spark I didn't bother changing the router because everything was working fine. This morning the modem went kaput (i.e. it won't power on any more), and I swapped in the Spark modem (HG659b) I got when I signed up. I had expected it would work seamlessly, but you can guess what actually happened -- no Internet -- and the helpdesk's diagnosis was "new modem required".
When I got home I decided to try connecting my laptop directly to the ONT to see if it would work. I found the PPPoE connection wouldn't establish if vlan tagging was on, but worked fine with it off. I unchecked the box in the Spark modem, and now the tubes are flowing again.
Is this expected behaviour? I had thought vlan tagging was required for Spark connections. I would love to go back and check if vlan tagging was enabled on the Bigpipe modem, but unfortunately that's not possible.
(As an aside -- the Connection Promise was fantastic. I borrowed a mobile broadband dongle from a family member and the helpdesk loaded on 20GB really quickly. Without it I couldn't have downloaded Wireshark and solved this. It's a good reason to be with Spark!)