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.
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.
* 5 Years BeforeWhat the hell, you've changed so much, Kouji...I see you
I'm coming now
Wait upOut front
Where are you?I've arrived!
I'm now at the station
Where are you?Yo! Long time no see! You haven't changed a bit, Ryuu-chan!