Over the years there have been many formats for music, movies, and games. Every few years a new format comes out, and people begin to get things they already have have copies of, but in the new format. This has happened with vinyl records giving way to cassette, and later cassette to compact disk. Today we see people do this with Blu-ray copies of moves to replace DVD copies.
While DVDs replaced VHS, they offered more than just the movie. A DVD gave you many special feature, with deleted scenes, commentary tracks, and so forth, things a VHS could never offer. Blu-ray is now replacing DVD, but other than a better resolution doesn't give much more. While a Blu-ray is great for a new movie shot in high definition, what is gained for an old movie? Does "The Godfather" get better with resolution, doubtful, or does a Saturday Night Live movie get any less shitty with resolution, also doubtful.
When the CD replaced cassette the CD offered better sound, and was in someways more durable. That is how a format change should happen, there should be a clear advantage for switching over. Today we seem to embrace change for the sake of change. A better resolution image is great, but is it really enough reason to buy something you had on DVD?