This one brings up some interesting questions. First of all, Natsu looks a bit older than 17 in this picture, which could suggest any lapse in time. Second, she's not using the dual kodachi. Instead, she's using a pair of short swords that are sheathed on her lower back as opposed to having her master's style from the last game. Also,
the other concepts depict her having different styles, like Vergeben specified. One style has her holding a bladed staff, the other has her holding a sword while powered by her demon. In any case, the reason is pretty obvious: She was changed to allow Taki to return using her classic style.
He said that PS would likely want to bring Taki back, but also salvage Natsu's character by removing her "replacement" status, which would be a start. After all, those who hated Natsu for being Taki's successor might be more willing to give her another look if she's in the game alongside her, not in her place.