By having Prosero become a woman, it changed the story to be a more interesting dynamic. Antonio, the brother of Prospera, now accuses her of killing her husband (The Duke) with witchcraft in order for him to receive her title. This then casts Prospera and her daughter out to see in exile.
As stated in the New York Times review of the movie:
"When the character is a woman, a central relationship in the play, between the magician and her doted-on child, Miranda, sheds some of its traditional, patriarchal dynamic. Instead, a mother-daughter bond fraught with envy, protectiveness and identification blossoms into something newly rich and strange."
It has been a long time since I have heard the story of The Tempest so I do not remember all the details enough for a proper comparison. Plus I haven't seen the film. However this movie has peaked my interest and I plan on watching it sometime soon. I guess what's so striking is how easy it was to change the gender of a character and how just that one move impacts the whole dynamic of the relationships seen.
Global Shakespeares article and trailer link
New York Times review of the movie