Diane Nelson spent the last few years running Warner Premiere, aka the company responsible for all the direct to video DC animated movies, so I'd be shocked if she knows nothing about DC. I mean, she may not know every character in the DC Universe, but who does?
She also was apparently in charge of handling negotiations with Rowling and the Harry Potter movies, which implies she knows what she's doing in terms of getting stuff adapted to film.
There isn't anything about this shakeup that screams that it's a stupid mistake and a lot of stuff which suggests it could be a good move. I suggest we wait 5 years before analyzing it to death.