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Could also be Kakakuridoji Ultimo, created by Stan Lee (but still a manga, because it was illustrated by I can't recall who, but he was/is a famous manga artist)> But I'd think you'd remember robot buddies/fights, so I doubt that's it. Also not sure about the reincarnation-related genderswap part, haven't read it really, just summaries and a couple preview chapters.
I don't remember the name either, but Ultimo is drawn by the creator of Shaman King.

It's a manga, and there's this teen boy who's been reincarnated, along with his friends and wife from his past life. He's the only one who remembers their past life, and the only one who knows who they all were. The twist is that is past wife is know his best (male, I'm pretty sure) friend, and his past best friend is now a cute girl!