Being completely honest, I really, really like this new group of girls. Partially because they are designed to be a unit from the get-go; it might be why I like Project Fairy and Ryuuguu Komachi so much. They just seem to belong together...
However, I get the feeling that they won't "replace" Mio. Namco does not, strictly speaking, have an obligation to Mio. But I doubt she'd be passed over for these newbies. As I mentioned earlier, Izumi, Sakura, and Ako were designed as a unit, unlike most other CGirls. The CG CDs are designed as solo singles, which the default groups (Project Fairy, Ryuuguu Komachi, and Jupiter; to a lesser extant the DS group) never get until after their group song. (Overmaster and KisS for PF, though Miki had albums before that; after RK was formed, Smoky Thrill came out before their newest image songs; Jupiter didn't get solo songs until the Jupiter CD, quite some time after the game's two singles.) I'm not saying they'll get a group song, but if they anything, it's likely they'll get a group song first. But even then, it'll probably not happen until after Mio's CD.
... And remember that this is all baseless and such, but it's how I see it. And I wouldn't even mind seeing them first, since their adorable and more interesting than the New Generation girls.
(PS, I really love the sig Marx. <3 )