Are you currently very active? Do you do any real fitness activities. I firmly believe that weight does not matter. While it can be a useful indicator, things like weight and BMI are rather useless.
Like you said you don't need to lose any weight, but you might find some benefit from exercise. If you start working out, lifting some actual weights (if its colored and coated in rubber, it ain't heavy enough, lol) and simultaneously bump up your food intake (healthy stuff of course) You will likely find yourself getting more fit, while not changing the number on the scale. Thats a good thing.
As far as how dramatic a difference it'll generate its tough to say, but I can promise you that if you work hard, put in the time and effort and evoke a change, you'll be a lot more proud and confident about your body than if you just stare at yourself in the mirror poking at your ribs
On a completely different side of things, and this won't really work with that sort of dress, but a corset, if used properly, can help give you a curvier appearance.