Wow, puberty hit her like a ton of bricks. A ton of ... really curvy bricks.

I think this is not an gender swap, this is all about before and after. The childhood friend was a girl all this time but she's just look tomboyish before until puberty hits her. Also the artist's commentary didn't mention anything about her being gender swap or not.

So I'm going to remove the genderswap tag.

Artist commentary doesn't mention anything because there is no commentary.

That said, the artist-added tags and title include multiple terms for genderswap (TSF, 女体化, 女子化).

Since Kouji is a male name, he must have been a man, at least previously. At the same time, probably no further explanation was necessary for the Japanese.