Maggie has never been anything but "British" to me, even when she tries an American accent (with very uneven results). It's as much about her personality and sensibility on screen as it is with the accent. She's a strong British presence, always.
Not that you couldn't change the character to British, but I think it would hurt the impact of this story. Sister Aloysius, and the entire plot of Doubt, is highly symbolic of the American repression of the era, and its lack of faith in humanity.
Actually, Maggie herself already understands this, apparently.
"Jaws is the Citizen Kane of movies."
blocked: logan2, Diamonds3, Hamilton22