Ahhh, body swapping. That tired old cliché that has been done hundreds of times in manga. Usually, we get a cheap laugh or two about the characters' failures at keeping up their façade around their family and friends, maybe a couple ecchi situations for the unsuspecting boy stuck in a girl's body, and a hackneyed romantic development to wrap things up before everything goes back to normal. And so begins this manga: with an elementary school boy and girl who swapped bodies after falling out of a tree and banging their heads together.
Given how well-trodden and mediocre the trope it builds its premise around is, I did not expect much from Shishunki Bitter Change. But against all odds, it manages to do something truly extraordinary. It is a series that asks in a completely serious tone, what would it *actually* be like to switch bodies with someone of the opposite sex? To have to live their life, not for days or months, but years?
This is not a comedy manga. The experiences of the characters main characters Yui and Yuuta are not played off as a gag, but are treated with heart-wrenching realism. The two suffer through actual gender dysphoria as they go through puberty and see their bodies changing ever further away from their image of themselves. They struggle with being separated from their family and the prospect of never being able to be their true selves ever again, of always having to hide. They become miserable when seeing the everyday life of other boys and girls around them, knowing they themselves can never have that. They feel distraught and disoriented when other characters treat them "as a boy" or "as a girl" when all they want is a friend.
If I had to sum it up, this is a manga about cisgender characters being made to live through the experience of a transgender person, the actual sensation of being "in the wrong kind of body". And it is handled excellently, with tact but with a level of drama and intensity that keeps you hooked. The plot itself is mostly slice-of-life, there only to give us more time to see the characters interact. In these day-to-day moments of the main characters' school lives we catch glimpses of the relationships between the characters as they shift and grow with the passing years. There are crushes, love triangles and confessions, but all of this is overlain by the suffocating fact that neither of the main characters can ever allow themselves to be true to their feelings or even their own identity.
Visually, the art style of it is relatively simplistic but clean, having been originally published as a webcomic the panels are arranged vertically and there are relatively few of them to a page. The characters emotions are well conveyed through relatively few words, for example several times in the series there are whole-page shots of either Yuuta or Yui simply looking at their appearance in the mirror and feeling uneasy with what they see which speaks volumes more than any text could have.
All in all, this is easily the best body-swapping manga I've read, and among the best drama manga I've ever read. I would strongly recommend you give it a chance.