@futbuk Technically, while the scenario you proposed looks the same as the premise of this manga, it's completely different since this is a magical gender bender.
While in the brain transplant scenario the person would be going through all sorts of mental anguish (Even if they stayed the same sex and wasn't given drugs, they'll still have heavy doses of body dysphoria since our brains has a specific map of our body that if it doesn't fit, can really screw people up (This is also the reason why things like Phantom Limp happens)), it's not really the same as what's happening here.
A more similar scenario would be taking the electrical impulses that make up your brain and putting it in a completely different brain in a completely different body. Of course, this gets into questions like "What makes you 'you'?" and other such questions (And that's only on the science side of things, if one gets into souls and such, that's a another giant can of worms waiting) that this manga would probably not get into.
So uh, yeah, what makes a person a person is very complicated.
Also, as for the beginning thing, that's just more a tiny suspicion on my part then any sort of confirmation on this manga. Really, nothing else has been really happening so far in terms of the protags character development to say anything is heading about it with even 40% surerity. Manga's like 10% Tachibana stays girl/6% turns back/17% TachiXJinguu/1% TachiXmystery girl/4% JinguuXAssasinGuy.