So, if the problem looks like a screen format mismatch, there's nothing to lose by fiddling with the settings. More, when the TV boots up, a fragment of picture flashes briefly before the BSOD appears. And it's not a true BSOD; it looks like an overscan problem.
Nothing ventured, nothing gained.
Menu > Options > System Settings > Aspect Ratio. Choose 1080i. The DVR asks to test. Sure. Tests okay. Keep Settings? Sure.
Punch the remote's ON/OFF button. Both TV and DVR shut down (noting that, unlike TiVO, the AT&T DVR stops its disk spindle). Wait a minute to ensure everything's really, really off.
Punch ON/OFF again. TV and DVR wake up. The picture flashes on the TV, then the screen goes black with a brief message 1080i. One second. The picture returns with HDMI 1080i in the upper right corner.
Joy. It works fine.
Now to call AT&T and tell Kristal #KW308G that the service call is no longer needed.