They're going to need a new Batman anyway:
1) Although JLM won't be released until after the third Nolan-Batman movie, it's production will probably overlap, so they wouldn't physically be able to use Bale.
2) Bale is contracted only for three Batman movies. He evidently doesn't want to do more than three, because WB had to scrap their Batman vs. Superman movie plans because it would class as Bale's third Batman movie (meaning Nolan would have to recast Batman). If Bale was willing to do a fourth, we'd have the film already.
3) This is a completely different Batman, and JLM is set in a completely different universe to Batman Begins. Batman Begins is set in our reality. It follows the laws of physics we abide by. JLM doesn't. Superman, Aquaman, Wonder Woman, etc. don't exist in Nolan's Universe. So creatively, you can't use Christian Bale as the Batman in JLM. It's a paradox.
People seem to have assumed Nolan's Batman is the same as Frank Miller's Batman. No it's not. Frank Miller's Batman Universe was still supernatural. It still had magic and aliens, just told in a grittier manner. That's why Bale would work in JLM if it was actually Frank Miller's Batman. But it's not, so JLM and Bail don't mix.
As there's no other Batman, they need to find a new actor. And this guy they've found is good enough. JLM is a reimagining of the Justice League anyway, so Batman being younger is one of the changes. Big fucking deal.
I think the only reason this isn't in production right now is because of Superman. I reckon they'll use the new Superman movie to set-up the rules and setting for JLM. Or vice-versa, and use JLM to set-up Superman. Fans don't realise the problems WB face here. JLM/Neo-Superman needs to be completely different to both Smallville and Nolan's Batman, without getting a fat-ass lawsuit by Siegel's family.