Why did Angelina Jolie replace Catherine Zeta Jones in the 2011 musical movie Cleo?

ok, catherine zeta jones was supposed to be playing cleopatra in the new 2011 movie cleo. i have hears rumors of angelina jolie replacing her, and i just saw a pic of what looked like her on the set of the movie. i would like to know why the hell she is replacing catherine zeta jones, or if she's even replacing her.