Blu-ray does add detail,
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This review is from: Harry Potter - Complete 8-Film Collection [Blu-ray]  [Region Free]  (Blu-ray)
Many films I cannot really tell the difference, but for example, on the first boat ride, the background seemed much sharper in blu-ray, not blurry like I remember it. And this has lots (maybe all) of the extras that used to be on separate discs.