I went back and looked at the trailer. It's definitely not the same dress, but it does seem to be the same color/texture. Maybe it's the lighting, but I think there's some significance to this. But then I've been known to hear hooves and think 'flying reindeer' on occasion.

Repost...

From what I got of the movie...

The green hospital/academy movement suit is worn during the escape scene, and the scene where River rememberdreams (I just used it, it's a word, okay?) walking through the school as everyone has already lain down and is then attacked by the Reaver.

River changes from the blue dress into a different green dress before they land on Miranda, but it isn't the movement sensor suit. A few of the characters change.

Hope that helps.

As for the suit, I always figured that, because Dr. Mathias refers to her movement trainers and her being graceful and whatnot, that they would serve the purpose of tracking movement (especially low key twitches and reactions while strapped to the chair), but yes, it does make more sense for them to be about keeping her strapped in.

I over think things. It's another case of hearing hooves and thinking zebras.

In the Visual Companion, there was mention of the metal plates being a method of keeping River in the chair while undergoing her "treatments".

I'll keep an eye out next time I go, movie people are pretty meticulous about making people in the background stand in the right places and little things like that so everything in the film flows from scene to scene so if her wardrobe suddenly changes then I'd imagine there was a reason.
If the gown with the little metal plates was from a hospital then they might serve for doctors to connect sensors to monitor her vitals but thats pure speculation. It might just be a fad.
-e

I must admit I want to know what purpose those little alumnium plates served. *shrugs*