Rei Rei Vol 1 (Gender Transformation)

Rei Rei is about a goddess of love called Kaguya and her servant Pipi who go around doing good deeds that will encourage love.

In Vol 1 of the OAV, Kaguya comes across a young man who is in love with a girl, though this love in unrequited as this girl is in love with a female doctor who she thinks is in love with her too.   The boy, in a fit of depression, tries to throw himself in front of a train, but Kaguya stops him and explains that the girl he loves is not with another man, but a woman.

Later, the doctor kills the girl.  For the boy to get revenge, Kaguya turns him into girl so he can seduce the doctor’s new lover and give her a taste of her own medicine.

There is also a hentai manga series that has a few gender transformation plots.