My goal is to be able to view the EPG from a new lineup through the web browser.

I moved my KM box to a new lineup: I have updated my Schedules Direct account to reflect this.
I added the new 'video source' and removed the old one. In addition, I bound the TV tuners to the new 'video source'. Finally, I ran mythfilldatabase and rebooted the machine.

The Channel Editor shows duplicate channels: one for each lineup. Tried deleting them all and rescanning. Now, there are no channels (doh!). Ok the backend was 'stopped' so restarted it. Rescanned, but still no channels.

Maybe running mythfilldatabase will help. OK pointed the browser at the server and OMG! it worked. I now can see the EPG data for the new lineup.

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R7.3: 0.22.20091023-1, Hauppauge PVR-500 (Philips FQ1236A MK4), Gigabyte Gigabyte EG45M-UD2H, E5200 2.4Ghz, 2GB RAM, NVIDIA GEFORCE 256MB

As your sig mentions you are using a PVR-500 you shouldn't be scanning for channels, you need to fetch them from your lineup(s). See http://knoppmythwiki.org/index.php?page ... WatchingTV