Pain and humility are the essential elements of Elizabeth's changes in The Crucible. Learning to let go of her pain and to forgive, humbly, is the primary driver of her outward change.
She has been emotionally hurt by her husband, John Proctor, when he has an affair with Abigail. Elizabeth remains hurt for quite a while as we see early in the play. She lashes out at John and holds his mistake against him, using it to both make him guilty and to push him to do certain things she feels are morally mandated.
When John breaks down and tells her that she is ill-treating him by being so cold and he begs her forgiveness, Elizabeth refuses at first to take responsibility for her behavior. Later, however, she agrees that her coldness was what drove John to his affair with Abigail and that the responsibility for that mistake includes her.
Her change of heart on this matter leads Elizabeth to defend John Proctor to the judges, lying to deny the affair. This perjury completes her transition from accuser to accused as Elizabeth breaks her moral code by telling the lie. She is no longer the hurt victim lashing out but a humble person acting loyally and proudly to protect the person she loves.
This change clearly moves Elizabeth from a position of powerlessness to a position of moral power.
The most notable change seen in Elizabeth Proctor is her view of her husband, John Proctor.
In the beginning of the play it is clear that she blames and hates John for his infidelity with Abigail Williams. Her words and body language shows how cold she is to her husband and John even demands that she stop being this way, since it has now been months since the affair. He hasn't gone into Salem in months, which is where Abigail is, in order to keep Elizabeth happy. As the play progresses, and the hysteria mounts, Elizabeth is accused of being a witch (by, surprise, Abigail). She is not hung immediately because she claims to be pregnant, which over time is seen to be true. The officials will not hang a pregnant woman until she has given birth. In their final Elizabeth admits to John he faults in the matter of their marriage and his affair. This admittance is her greatest change. She acknowledges herself as a "cold wife" and that is was this fact that led John to find warmth elsewhere. She admits to John that she never thought a man like him could love a woman like her, and this led to her coldness.
So, simply, in the beginning Elizabeth blames John completely for their marital problems and in the end she recognizes that she had a part to play in it.