Quick summary:
Sayaka is bed-ridden and dying in hospital, but she possesses popular girl Sakuragi Erika and uses her body to be with her beloved boyfriend Naoto. But she has yandere tendencies. When Naoto jokes that maybe she's not really Sayaka, she decides to prove it by stealing his body and putting Naoto in Erika's body. Her power allows her to overlay a person over the original owner, inheriting their memories and such. Furthermore, if a person orgasms in that new body, they forget their original self. She then forces Naoto into becoming Erika, and later many other girls, including his own little sister and mother.

Sayaka reveals that she both loves and hates Naoto, jealous that he can run around healthily while she's dying in hospital. Her original body later passes away, while she continues to possess Naoto and other girls. Naoto later uses willpower to remember being himself, and eventually realizes he enjoys being a girl. He asks Sayaka to conspire with him and take over various girls' lives, to which she obliges.

Last I checked, it's currently 5 chapters (chapter 5 description says Sayaka has something nasty planned for Naoto), and IIRC Marialite said there's a finale in the works.

Base +6, ensane +9

@failbags
Your summary is pretty accurate, but there are a few finer points that are slightly incorrect I think:
Sayaka apparently has the power to either allow the swapped person to "coexist" with their personality AKA read all their memories, or she can suppress them so it appears like they aren't swapped at all. (Although still having some personality traits/tendencies under the surface from the swapped in person)

It's not until she swaps Naoto from the little sister into the mother that she notices Naoto is retaining memories from every subsequent swap, but isn't thinking straight, so doesn't really think much of it unless Sayaka specifically points it out. At the moment of climax, she swaps Naoto out of the mother back into his own body, which is the point he regains awareness. She swaps him back into the mother a short time later and explains he needs to concentrate on his sense of self and not get overwhelmed by pleasure or he'll lose his personality again. After that, he's generally able to maintain his personality, albeit with moments of weakness where quirks from the original body's personality leak through.

Then in Ch3 Sayaka helps Naoto take Erika's body again: She swaps him into Erika while putting Erika in Naoto's body, but sealing her personality so she thinks she really is Naoto. At that moment is when Erika's older sister comes home, leading to a scene wipe to the next morning. We find out Naoto was swapped into Erika's older sister, and Sayaka is in Erika. Naoto's mother was "released" and so she has a puzzled look while she takes "Naoto" home with her.

Sayaka then thinks to herself that she had a change of heart about Naoto: Seeing him so happy the previous day made her realize she's never been happier than seeing Naoto being so happy. So she was now going to dedicate herself and her powers to making Naoto happy and letting him experience swaps with whatever women he likes. Naoto comments that he really loves the older sister's body and that it's the perfect setup for them then: They both live together, so they can see each other all the time and can do whatever they want with each other outside of school, etc.

Base +6