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I don’t remember if it’s a hentai manga or not, but there was one where a boy could change into a girl whenever he wore her clothes - so he turns into the girl whose clothes he is wearing. In the end he buys a completely new set of clothes online and turns into a female version of himself. He then meets up with his best friend who is crossdressing and during this encounter he decides he wants to live the rest of his life as a girl.

Edit: I found it. For anyone interested, it’s called “Metamor ★ Coordinate 2 ~Ane no Yuujin Hen Nyotaika Hen~“