I know this is old but I'm SO GLAD I found this!! I got off the phone after 30+ minutes with a technician who ultimately said it must be an issue with my tv. I knew it wasn't and they couldn't answer why the instructions in the RNG200 User Guide (from their website, btw) weren't working. The guide says:
• Make sure the set-top is Off and the TV is On.
• On the front panel of the set-top, press MENU and INFO at the same time.
This does nothing...but simplying pushing the Menu button on the remote did.
Press "SELECT and CH+" on the Cisco RNG200 box until you get the HD desired.
Why did that have to take so long to solve? The agent I talked to also said you can change it by remote control by pressing the menu button twice and there is a selection for cable box setting which will allow you to change it. This was not an option last I checked but now I'm getting some kind of update so maybe it will be now. In anycase, I can't check now because the Cisco box says "Au:Ld" and doesn't work. I used to have a service that didn't have any of these problems whatsoever. If it wasn't for the wacko HOA's that don't allow you to put anything on your roof I don't know how Comcast could still be in business FYI (i.e. frustrated).
My HDMI box went from 1080 to 480 and I didn't change anything. The Comcast rep came and fixed a setting and it changed to 1080, but that only lasted for 2 days. Good thing I watched the guy change the settings and paid attention to how to do it. If you turn off the cable box and leave the tv on, then hit the menu button on the Comcast remote this will bring up a white menu that will give you an option for resolutions. The thing I learned also is the reason why it changes is because Comcast HDMI cables are pieces of garbage and after a while they won't even read an HDMI 1080 resolution.bottom line is, order a new HDMI cable (even the cheap cables on eBay are great) and change the settings to 1080 and you will never have this issue again!