It is interesting to compare Madame Loisel before and after the loss of the necklace, because there does not seem to be much difference in her character; as a whole, Madame Loisel's changes are merely superficial for she, as the first part of the story explains, is naturally built to feel as if she deserves better than what she has. Keep in mind, however, that Madame is not a round character, so be careful not to confuse those changes as transformations.
Natural ingenuity, instinct for what is elegant, a supple mind are their sole hierarchy, and often make of women of the people the equals of the very greatest ladies.
Therefore, the tragic flaw of Madame Loisel as a character is that she was born to feel different than others; she is basically misplaced in station as well as in social rank. This being said, we can safely conclude that Madame Loisel is not meant to change so much in the inside despite what goes on in the outside of her character.
One thing that can be asserted is that Madame Loisel starts the necklace experience feeling unhappy, then hits the ultimate sense of achievement during the party and, as she notices that the necklace is gone, she is sucked down into despair.
Before the necklace, she was
unhappy as if she had really fallen from a higher station; since with women there is neither caste nor rank, for beauty, grace and charm take the place of family and birth.
This means that any change in her character would have to come from a change in her superficial life; Madame Loisel will not change on her own.
During the party, and as she wears the necklace, her joy reaches a very high point. This is not a change in her character, but a change in her mood due to a shift in her situation.
She danced with rapture, with passion, intoxicated by pleasure, forgetting all in the triumph of her beauty, in the glory of her success,
Notice how it is her beauty and success what bring about her happiness; yet, beauty and success are both temporary accidents of life.
Finally, the loss of the necklace conveys a similarly superficial change in Madame Loisel; she is still unhappy, she is still dissatisfied. The only thing that she can think of is how to replace "the ornament". The loss of the necklace has already aged her and made her rough and impoverished to the extreme
Madame Loisel looked old now. She had become the woman of impoverished households--strong and hard and rough. With frowzy hair, skirts askew and red hands, she talked loud while washing the floor with great swishes of water
All of these are merely superficial changes because, shortly after, the narrator explains that Madame Loisel still thought about the ball, and about her night of glory, whenever her husband was not around.
Conclusively, the only positive change in Madame Loisel is the fact that she complied with the duty of replacing the necklace, although that was still a somewhat non-altruistic feet for, a) she had to do it, regardless and, b) she felt "heroic" about it, as the story says.
In all, Madame Loisel will demonstrate to only be able to change her outer life while she, deep inside, still wishes for bigger and better things.