Leonie from These Old Shades doesn't change in personality. She does transform from girl-dressed-as-boy to girl-dressed-as girl. Justin Alastair, Duke of Avon immediately identifies her true sex. His friend, Hugh, takes a little longer. By the time Alastair delivers Leonie to his sister, a number of people in Leonie's life have already seen through his disguise.
That is, the transformation is not like Viola's in Twelfth Night, the ultimate revelation that she actually is female. For Leonie, giving up her disguise means she must accept a change in roles.
Leonie weeps bitterly when Alastair gently informs her that she will stay with his sister and be introduced to womanhood. In the long run, Leonie is comfortable with the transition. In the short run, Heyer captures the pain of change, whether due to circumstances or age.
Heyer's triumph is that Leonie remains herself: honest, active, a little bloodthirsty, and utterly devoted to Alastair. She learns to dance. She enjoys "dressing up." She learns that she is pretty and enjoys her prettiness.
She sheds her disguise when she accepts the inevitability of change. She does not shed her fundamental character.