The biggest broadcaster blocs that use 720p include ABC/Disney/ESPN, Fox, and the A&E family, while 1080i is used by NBC/Universal, CBS, Time Warner, and Discovery. But the caveat is that many older 720p and even some 1080p models cannot handle 1080p signals at all. It will be upconverted with a simple conversion, causing no side effects. Here is information collected from the AV forums about what broadcasters do what. Assuming the TV can accept a 1080p signal, it will be scaled to 720p. You can show a 720p source image on a 1080p TV without any problems.

If that doesn't work then try the advanced menu setting and select native mode, which means your tv will control the video settings. Things To Consider When Deciding between Resolutions for Your Stream. I took that to mean the TV was upconverting the signal, but that is correct, FiOS broadcasts what the broadcasters output and they output 720p for some stations and 1080i for others try verifying the 1080 setting in the dvr, by clicking menu>settings>video settings and then selecting video format. While the 1080p video received a score of 73.1, the 720p video received a score of 76.8meaning human viewers would actually prefer to watch the 720p video rather than the 1080p video.

My tv is 1080p but shows 720p