My old WDTV Live Plus had this option. While watching a video, you could change the video playback format from “Normal” to “Widescreen”. Now I have to stop the video, go the the Settings menu, then Audio/Video Output, then Aspect Ratio.
I have a mixture of “old” 4:3 content and new 16:9 content. Having to keep going through the menus to change this is cumbersome.
If no shortcut exist, this can be solved by:
One of the colored buttons on the remote cycling through the aspect ratios.
Adding an option in the Option menu of the current video that’s playing.