I doubt we can say for sure that she is replacing Bond permanently. Daniel Craig is done after this one, I'd say (he didn't even want to do this one), but every time a Bond actor moves on they always more or less reboot with the next actor, different take and all.
I don't know how this movie resolves although it would seem like it would have to end with her still as 007 if Craig's out. If her character is well-received, they'll make more with her as 007, I'm sure. But I can't imagine they'll go too long resisting the urge to bring Bond back in another reboot (Elba please!!), presumably in a separate "universe".