After Mrs. Gargery was seriously wounded, Biddy came to live with the family. Pip had the opportunity to observe her more closely now. The narrator Pip tells the readers that he could sense a change in Biddy. Her shoes changed to heel, she was more car

Biddy does act differently from the teacher she had formerly been.  She showed how caring she could be in working with someone so difficult as Mrs. Gargery was; she brought a sense of peace, calm, and organization to the home which had not been there formerly.  Pip may very well notice her more as a woman and less as a teacher or another acquaintance; she might be changing her personal characteristics to impress someone else.  Since we later learn that she marries Joe, perhaps she is becoming more attractive for his benefit.