There are several characters who experience significant shifts in power in the play. Abigail Williams and Reverend Hale present the most compelling examples of characters who change in relation to the possession or attribution of power.
Abigail is nearly powerless at the opening of the play, gossip has gotten around about her being fired from the Proctor house and she is accused of dancing naked in the woods. From this position Abigail rises to prominence and influence, ultimately playing an instrumental role in "proving" the guilt of both John Proctor and Elizabeth Proctor.
Reverend Hale arrives with a significant moral authority at the beginning of the play only to lose that authority entirely when he realizes that the witchcraft claims are false. He further slides away from any moral authority when he fails to speak out publically against the trials.
...he lacks the moral conviction to act against proceedings that will condemn innocents to death.
I would argue that Reverend Parris experiences the most intense shift of political power in the work. On one hand, Parris represents the pinnacle of power. At the time, Salem's political power rested with the spiritual authority. Therefore, as the town's reverend, Parris enjoyed a great deal of political power. This is seen in the trials and the discourse leading to it. By the time Act III unfolds, Parris is massively powerful. Yet, in Act IV, the vision of Parris that is rendered is one where there is complete and utter disintegration. Parris understands that his position in the Salem social configuration is rapidly descending, something highlighted by the death threats as well as the lack of public support in the trial. At the same time, Parris understands that he is no longer seen as a spiritual force as the trials are becoming more transparent as a political exercise that has lost legitimacy with the public. Miller's epilogue to the drama, "Echoes Down the Corridor," reveals this shift and transformation as complete. The ending of the trials results in Parris leaving Salem "on the open road" and never being heard from again. In this, Parris experiences the greatest amount of shift in political power.
yes but what about hale, isnt doesnt he change the most in the play.
Personally, I think it's Procter. At the beginning of the play, he was utterly powerless in his situation. He was trapped by the power of the sin, his shameful affair with Abigial.
At the end of the play, he find power. He confesses to his sin "I have made a bell of my honour!" and is freed from it. He is powerful. By confessing, he has the power to prove Abigail's true colours.
True, the sacrifice he had to give - in order to retain his name - was to be hung. But, he died in a powerful position. He "had his goodnss". He died redeming himself, going against the court.
He died a powerful man.