Those are some nice words, Euphy. However, you only know the Anis who already had memories of a past life. You don't know the Anis before she got them. So is Anis the same? Were there two personalities that merged together? Did og Anis die so current Anis could live? We'll never know.
Well, in the very beginning of the manga, it says she 'regained' her memories, and shows a scene of her as a child lamenting that she couldn't use magic, before regaining her memories, to such a degree that it, in itself, triggers her to regain them -it's outright stated that it was like finding a crucial piece of herself that had been missing. Then, in Chapter 44, the Spirit Contractor goes on explaining that the core of why she's never been able to use magic is because unlike others in the world, her soul doesn't contain any spirits, and therefore, cannot contact the spirits outside her to trigger magical phenomenon of her own accord.
She has
always been Anis, from the very start, soul-wise. Since the general belief is that the core of one's personality comes from their soul, it can be assumed that her personality as a child before then was generally the same, only... y'know, a child, inexperienced in the world, save for what they've been taught and seen themselves. So I'd say it's a case of merged experiences, with the previous life having more weight initially, due to having lived longer at the point the memories were recalled. But ultimately? She's always been herself. I understand the worries of 'replacing the original body holder', though.