The biggest change in a character here is in the character of Koly. This main character changes from a sad, insecure widow to a confident woman who is happy and secure in her fate. Koly's change in character happens because of her love for Raji.
At the beginning of the story, Koly is a sad character, indeed. She is promised in marriage to a very sickly man whom she is forced to marry through the will of her parents. It isn't long after the marriage occurs that her husband dies. In the Indian culture, this forces her into a life of mourning. There is not much hope for another husband when a woman becomes a widow. Koly knows this and grieves.
This is when Koly's change begins because she meets Raji. Even though he is only a rickshaw driver, Raji greatly respects the woman Koly has become and values her skill at quilting and embroidery. The two fall in love. Koly finally is able to admit her feelings as Raji proves his respect for her in working hard and building a home where he builds an embroidery room for her. The story ends with their beautiful engagement accepted by her beloved Indian culture.
In conclusion, it is appropriate to name the exact "how" and "why" of your original question. How does Koly change? She changes from a sad widow to a happy fiance (all in the midst of her Indian culture). Why does Koly change? She changes because of love.