I disagree. While the original KK DVD was excellent, the HD version is noticeably clearer and with more color depth.
Viewed on 60" RP in 1080i with Xbox360 HD-DVD vs 480P (with Faroudja DcDi upconversion to 1080).

I point you to several scenes to check.
1. the filming on the ship scene - in the DVD, the sequins on the dress are discrete only in close-up, in HD they remain distinct in all shots. The sky too seems to have more colors in it. Quite pretty. Look whenever the camera pulls back, the DVD goes soft and fuzzy while the HD stays sharp (except for camera focus errors, which show up in the HD)

2. Kong on the mountain with the girl/sunset "beautiful"... again, in the HD version, kong's hair can be seen individually moving in the wind in the wide shots, not just the close ups. Also, the color depth between light and dark is higher. Sky colors are quite nice.

3. The night time NYC Empire State scene. In HD, the windows in the far away buildings are distinct, as are the head lights of the cars below. In the DVD, the far building lights are blobs and the streets show no details of cars.

All in all, my wife and I thought the difference was quite apparent.