Blu-ray, true high definition?

I have recently bought a nice little blu-ray player, along with a couple of blu-ray discs. These are discs of relatively new programs and look the part, much more detail and a much sharper image. However, when I was looking in a video store, I saw a lot of older movies (such as close encounters of the third kind, the original terminator etc.) On the box, it promises a high defintion experience, but surely these films are recorded with older, non high defintion cameras. Is there any point in paying extra to have these films on a blu-ray disc when I can buy a standard DVD for a 3rd of the cost or do they do something with the video to make it high defintion? It seems a bit of a rip off and false advertising if its exactly the same video just burnt onto a diffrent kind of disc.