Looking for non-isekai reincarnation manga where a boy gets reincarnated as a girl

Not reincarnation per se, but the webcomic Misfile may be something you’re interested in. Stoner angel accidentally misfiles a dude’s paperwork into the “women” cabinet, and accidentally loses two years of paperwork from another person, then promptly gets banished to earth so he can’t fix it. So this dude wakes up the next morning as a woman, and the other person wakes up missing two years of their life. From that point, it’s basically a high school comedy.

It apparently started because the (male) author preferred writing guys but preferred drawing women. So his natural solution was to put a male character into a female body. But apparently it resonates pretty hard with a bunch of trans men, because the main character often struggles with bad body dysmorphia as a result of the misfile.

It started back in the mid 2000’s, with the author posting a page per day. It spanned like 30 books and ended sometime around 2020. It’s also interesting to see the art style develop as the series progresses, because the earlier stuff is basically akin to pencil sketches.