A lot of misunderstanding about this, both by those who hail it as a great milestone and the minority of right-wing fans who denounce it. She's NOT going to be playing James Bond. She's playing a different agent who uses the same code name, while Bond is out of action for some reason.
Perhaps what this really means is that MI 6 has finally figured out that it may not be a good idea to have the same operative use the same code name for dozens of missions over many years - especially if that operative is as highly conspicuous as Bond alway seems to be. Any enemy who isn't completely brain-dead has long since figured out that 007 is James Bond. Having a different agent use the code name might confuse the enemy - at least for a time. And perhaps she can avoid leaving a trail of explosions, bodies, and jilted women everywhere she goes, and thereby not attract as much attention as Bond does.
Bond tends to forget that the whole point of being a secret agent is.... secrecy! Perhaps the new 007 will do better.