Fear no more, Potterheads.
According to THR, Rowling will write the screenplay for the first film all herself, and if WB wants another screenwriter to rewrite it, Rowling will have to approve the process.
As for the rest of the films in the series, Rowling will have script approval, so even if she is not writing the film herself, she will ultimately dictate the direction in which the series goes.
It seems that Rowling’s full vision will be realized for this film series.
Source: Movies Blog (MTV)