Ideally the weight of a kilogram defined by the new definition should be identical to the weight of the old physical "the kilogram" object. The goal of this isn't to change how much a kilogram weighs, it's just to change how it's defined.

For a comparison, it used to be that we measured the speed of light as being 299,792,458 meters/second. Since then, we have defined the meter to be 1/299,792,458 of the distance that light travels in one second. An object that was 1 meter long before is still 1 meter long, but the way we communicate how long a meter is has gone from "it's how long this particular stick in France is" to a precise definition based on fundamental properties of the universe.