Yes, finally – Seinfeld is now in High-Definition! Well, unfortunately, there is some bad news – it is not really high definition. While many stations believe that high-definition is widescreen, it is not. Basically, they managed to get the old footage that was still in film (as film has a higher resolution than HD) and then cropped it.
The technical term for this is called upconverting, which means it basically converts it to a higher resolution. So expect it the picture quality to be a bit weird.
You can view the original DVD one here, and then compare it to the one above.
Source: AVS Forums