God was running kinda short on time, so he decided to go with the simplest option so He could go rest- He would turn Adam into Eve, and then work on a new version of human who He could prevent from sinning. All He needed to do was instill the seed he had planned to modify Adam's rib into Eve into Adam himself, so He did that and then went.
And so, as he slumbered, the seed took root in Adam and he became she. Where his muscles had been large and firm, they became lithe and dexterous. His handsome masculine form shifted to a beautiful feminine form. Upon his flat chest grew soft breasts on which her children would suckle. His hips and hindquarters widened to safely birth her young. And his penis receded and his testicles converted into ovaries, his scrotum giving way to her vagina.
The seed of humankind was now the womb of humankind, and with her form's changes came her mental changes. Once she awoke, she would call herself by 'she' instead of 'he', and forgot ever being Adam- she was Eve. Her instincts had changed too, as instead of desiring to impregnate and spread his genes, she wished to be impregnated and bear young.