Mary changes her story because she is afraid that she will be accused of witchcraft. She realizes that the judge is putting pressure on her to stick to her story. The judge has realized that he may have been mistaken on the idea that witchcraft is so prevalent in Salem. He realizes that the girls may be lying. If he agrees to hear Mary's story, he realizes he himself will look like a fool. He is more worried about his personal reputation than he is at finding the truth of the matter at hand.
Mary is torn between doing the right thing and saving her own life. She determines that her own life is more valuable than the truth. She does not want to hang. She changes her story in order to survive. She is afraid of Abigail. She fears what she can do to sway the judge against her.
Mary gives in to the pressure of those around her who are trying to prove that witches are among them.
As was mentioned in the previous post, Mary Warren changes her testimony because she is afraid of being accused of witchcraft. Toward the end of Act III, Mary Warren admits to Deputy Governor Danforth that the girls were faking in court. When Danforth calls for Abigail Williams, she confronts Mary Warren face to face. Mary is a rather weak, timid individual who succumbs to peer pressure easily and cannot maintain her composure when Abigail begins to act like Mary's spirit is attacking her. Abigail pretends that Mary's spirit is threatening her, which makes Mary begin to panic. Mary begs Abigail to stop, but Abigail continues to pretend she is being attacked, and all of the other girls join in. Mary realizes the judges have faith in Abigail and will believe her, so Mary does what is best for herself and switches sides. Mary begins to accuse John Proctor because she fears that if she does not, she will be accused of witchcraft. Danforth will think Mary tried to overthrow his court and severely punish her. To avoid punishment, Mary capitulates to Abigail and begins to accuse John Proctor of working with the Devil.