When a character changes personality half-way through a story

Often enough in a story, the personality of a character develops in a gradual process. Or they may undergo very sudden emotional or lifestyle transitions which leave their basic personality unchanged. But what about characters who undergo a sudden and radical change in personality?

I was trying to solve this problem for one of my characters when I happened to read Terry Pratchett's YA novel, A Hat Full of Sky. In it, eleven-year-old Tiffany Aching is taken over by a hiver and well... she changes. She (that is the hiver) keeps all of Tiffany's memories and her goals. It 'thinks' it is Tiffany, while the real Tiffany becomes a tiny shred of personality buried deep inside it. So, I made a list of all the things Pratchett does to convey this:

• Before the hiver takes Tiffany over there is plenty of foreshadowing, so we know what has happened, we just don't know how it will manifest itself.
• Right from the start, we learn that Tiffany feels different - better actually. Later on, we see that all her negative thoughts, while they aren't entirely new, have become more extreme. She is more self-confident but also harsher towards other people.
• She starts making little behavioral choices we had seen the real Tiffany reject not long before - nothing very important at first, but her new choices are generally more shallow.
• Soon, the false Tiffany shows a willingness to engage in rude, arrogant and even violent behavior she would never have contemplated before. As with her thoughts and feelings, these are entirely foreign to her. Instead, they are the most extreme manifestation of the little ambitions and resentments she had previously repressed. The hiver Tiffany no longer has any moral boundaries at all.
• While the humans around Tiffany don't notice the change at first, the spiritually sensitive goats and poltergeist know and don't want to be around her even before her behavior changes.
• Her reflection sometimes shows her the hivers past identities, including prehistoric animals. At times, the memories associated with these identities threaten to take her over.
• The real Tiffany finds a way to fight back and manifests herself.
• Eventually, the narrator gives an explicit explanation of what is happening to hiver-Tiffany and real-Tiffany.

We only see hiver-Tiffany in action for two chapters out of fifteen and it's a rather intense experience. I wondered how long it's possible to keep up this kind of wholesale personality change before the reader's relationship with the 'real' character becomes compromised or replaced with a new relationship. Interestingly enough, my alternate personality character, Anat, is also present for about an eighth of a book and I was glad when the old one returned, even though the new one was, in some respects, a better person.

In other respects the way I manage Anat's change is quite different. She doesn't experience a possession, rather she undergoes a kind of controlled amnesia. She foregoes her memories, substituting a memorized record of her life, much as we might learn the life of another person from a very detailed biography or diary.

• As with Tiffany and the hiver we see this process getting set up, but as with Tiffany, we don't know how it will manifest itself. 
• Because Anat is the narrator, I had more opportunity to show change by altering the narrative voice. The old Anat constantly reveals the depth of her memories in the way she sees the world, through metaphor, allusion and story. I tried to show the relative shallowness of new-Anat's experiences by paring the imagery down to a minimum.
• The new Anat has excellent recall of her past, but she's less biased and quickly understands things about her companions while remaining less affected by them than she would have been. Considering the old Anat is quite morally ambivalent at times, the new one has a greater tendency to think in moral terms rather than through her own feelings and reactions.
• The new Anat experiences time quite differently from the old Anat. She moves through it faster for one thing, and with none of the tendencies to digress into memory, fantasy, or expectations of the future which the old Anat had.
• Changes in Anat's magical abilities reflect these changes in her personality in ways that move the story along. The old Anat could see into the spirit world but paid little attention to everyday people. The new Anat has no visions, but does have an interesting ability to 'hear' people's stories, whether they were planning on telling them or not. This reflects her less biased, self-centered nature as well as the loss of depth.
• Since it's important that Anat's change is never discovered by her companions, there had to be a lack of obvious changes in her behavior towards them.

Described like that, my alternate character feels a lot more subtle than Pratchett's but in both cases, the change is a logical consequence of what has happened to change her. Many differences stem from the fact that my character is the narrator and Pratchett's is not, but they also come from the fact that my character is very much living a double-life and needs to retain it, whereas Tiffany's identity and abilities are fully integrated into her social world. Oh, and the real Tiffany is distinctly 'good' whereas hiver-Tiffany is very bad indeed. With my character, these tendencies are less extreme, but they're also reversed.

I'll certainly be interested to hear what my beta readers think of the old-Anat, new-Anat change.