I remember reading about one but you explaining It out was a bit confusing. Well what I've read is that there are generations of the human hero and the demon king and I'm not sure if that's what you're looking for.
Following
I'm not sure cause haven't read them but could it be one of those?
http://www.mangago.me/read-manga/magical_change/
http://www.mangago.me/read-manga/ryuu_no_yuigon/
i'm reading both so sorry, but both nope.
>magical change - highschool boy sorta conned to become a mahou shoujo who is chased around by the villain who had hots for his female persona
>ryuu no yuigon - BL high fantasy setting with the maou himself as main character, no gender swap.
Sadly no :( Thanks though
Asked this earlier but I'm going to ask again~~ I'm not sure if it's in this website or another but I read a yaoi a while back about a human hero that got turned into a female demon by his former best friend that turned out to be the demon king he was suppose to destroy.