What we’re talking about it neither stretching or cropping or black bars. It’s re-creating the image that would be there if a wider aspect ratio camera had been used to shoot the scene in the first place. In a sit-com, all the action takes place on one or two sets, which rarely vary, under consistent lighting. Over the production life of the sit-com, the camera pans across the set as characters move from place to place. You store all that information and you can build a digital representation of the entire set.
Something similar was used to shoot all the scenes involving Andy Sirkis as Gollum in the Lord of the Rings films. He played the part live on the set in a skin-tight suit interacting with the other actors. Then they shot the scene again without the actors, but with exactly the same camera movement. Then they were able to digitally eliminate Sirkis, replacing him with the background that he was obscuring. Then they could insert the smaller Gollum in his place.
Since that film, they have made a lot of advancement in automatically figuring out the 3D structure of a scene from footage shot with a single camera. So they are able to build a 3D virtual set that a person can interact with - passing in front of some elements, behind others. Other work has been done in extracting a person from any background because the person moves and the background doesn’t.
If you couple all these technologies together, you can - with equipment and techniques currently available - build a replacement for the sides derived from the original set. The end result can be seamless.
In fact, I’ll be surprised if something in this line is not shown at either the upcoming National Association of Broadcasters show, or at least the SIGGRAPH (computer graphics) show after.
“My Name is Earl” used to put various jokes & props in the HD-only edges of the screen. In one show, Randy was holding up a piece of paper that said “High Def Rocks!” In another, there was a beer tap labeled “Hi Def Ale”.
A reasonable compromise might be to do something like SNL does when airing a mix of high-def and old SD 4:3 segments - For the SD segments, they letterbox the image with a background that’s out of focus but looks vaguely like the foreground, which works surprisingly well. Or, they’ll letterbox it with a box that has Christmas Wreaths around it (for the Christmas special) or some other thematic letterboxing, which makes it look more like the show was intended to be presented that way.
You could do something similar with old sitcoms. For example, if you have a wide shot of Samantha’s house from one of the standard camera locations, put it out of focus and use it as the background, then overlay the 4:3 image in the center. For outdoor scenes, you can use a similarly-colored background (snow, trees, whatever), and use that as an out-of-focus background.
I don’t know if a skilled technician/artist could pull it off and make it look integral with the show and more pleasant to look at than black bars, but it would be easer than trying to splice in fake CGI backgrounds. And it would be a hell of a lot better than cropping or stretching.
AMC shows some “HD” movies that are clearly 4:3 standard def that they stretch using a progressive algorithm so the stretching increases as you get near the edges. I hate it. In fact, on my 92" projection screen, it’s unwatchable. If the camera pans, you get this weird ‘swimming’ effect that actually makes me nauseated. If someone walks off camera, their heads become strange stretched blobs just as they leave the edge of the screen. It’s horrible. I’d much rather watch the show letterboxed.
The Disney animated classics Blu-rays have a feature that adds artwork to the sides of the screens to fill up the space, like curtains or woods. It almost has the effect of extending the scenery in some scenes. I prefer the plain black, though.
Just wanted to add about Seinfeld, it was shot on 35mm film, so the original footage is slightly wider than what originally appeared on TV, plus they do crop it slightly on the top and bottom to get the widescreen HD image. It looks much better than when they try to just expand the sides, stretching the image.