I just got a copy of the Twilight Time edition of Christine and the colors are a lot more vibrant than the other versions released on DVD. I believe Twilight Time was the first Blu-ray release of Christine. So how do cable channels get a DVD-like transfer and put it in HD? Yes, the aspect ratio is off, but I believe that's the only difference.
Taking Christine as an example the original film format was 35mm which is, essentially, better than HD quality.
Film is analog so there are no real "pixels." However, based on converted measures, a 35mm frame has 3 to 12 million pixels, depending on the stock, lens, and shooting conditions. An HD frame has 2 million pixels, measured using 1920 x 1080 scan lines. With this difference, 35mm appears vastly superior to HD.
In this case TT have obtained a newly-mastered transfer of the 35mm film in HD format from Columbia-Sony
Christine is presented on Blu-ray courtesy of Twilight Time with an AVC encoded 1080p transfer in 2.39:1. Columbia-Sony continues to provide exceptional looking high definition masters to Twilight Time
Basically, the original release may not have been in HD ( not available in 1983) but with 35mm film a new HD master can be obtained any time.