It's not quite that simple. They're not really redoing it as such, but instead creating a new film based on the original comic stories.
One thing that was notable about Batman Begins is that it interpreted the comics in a very different way to the older films, and while some stuff was similar the backstories of just as many things were completely different. Indeed, one of the biggest differences is how exactly Gotham City itself became what it is, and how Bruce is intertwined with that. I suspect it'll be the same in Dark Knight, with the origin of the Joker, his motives, etc, being incomparable to the older Batman film.