Firstly, I'm not going to review the film at all as most people know the film quite well and probably already own it on VHS or DVD. As far as I can tell, it's the same film as on the DVD and no cutting or editing has been done to the length of the film or scenes, etc.
However, the Blu-ray image quality is significantly clearer and improved. By comparison, the DVD looks like a it came from a VHS tape! That would be the main reason to replace your DVD with the Blu-ray. At this price, I believe it's worth it.
There are a few more bonus features on the Blu-ray as well. However, the bonus stuff that is on the DVD and the Blu-ray is edited DIFFERENTLY, so some interview footage is missing or shorter on the Blu-ray. That would only bother the fans who want it all. Personally, I prefer the Blu-ray editing of the bonus footage anyways.
Others who are more passionate and intimate with the film than I have already posted discrepancies in the audio and music - the songs have been beefed up sonically they say. They admit it sounds stronger but don't like it that the original music and mix was tampered with. That's understandable. Myself, I was able to enjoy the film without being distracted by any noticeable changes. However, there is no 2-channel stereo audio mix, so I had to listen to the film's surround mix even though I only have 2 speakers. This is the reason I deducted one star. I can only assume the mix sounds good with a proper surround set-up. Also, there was one song where a noticeable amount of reverb was added to the vocal for part of the song.
A "funny" final point. My wife has loved this film all her life. We watched the film with the subtitles on, so she could see what the real lyrics to the songs are. It was shocking how dirty the songs (and the film in general) actually are and how much got past the censors! Be warned, if you're watching it with kids (or your parents) DON'T turn on the subtitles!