Transformation is an arguable point in Much Ado About Nothing. Ultimately, the good characters stay good and the villain remains bad. There are, however, a lot of little changes that occur along the way, as the characters learn (and mislearn) about themselves and each other. The largest transformations are characters moving from hating each other to loving each other, and vice versa. It’s also important that the characters begin their metamorphoses as a result of trickery or deception, both the mean-spirited and well-intentioned types. The deception tends to cause strong emotions, which ultimately drive the transformations. The transformative emotions range from shame to anger to affection.
Transformation is a plot device in the play, not a realistic portrayal of any believable or meaningful changes in the characters.
Masks are present as a potent symbol in the play because none of the characters have truly revealed what or who they are. The whole notion of "transformation" is just a misinterpretation; characters aren’t changing, they’re just revealing more of their true selves.