Some of life's greatest discoveries happen purely by accident. When Harry fell into the Lion's enclosure and was attacked, he knew that was it. Imagine his surprise to suddenly find himself in Jenna's body. As he looked on at the carnage in disbelief, it suddenly dawned on him that he'd somehow survived.
So a spontaneous accident had caused his latent physic powers to project him and possess the nearest living body upon his death. Harry had turned from a 57 year old cage cleaner into a hot 17 year old high school girl via an uncontrolled possession.
It would be weeks before Harry learned about the term body hopper, and what his ability might entail for him long term. He managed to work out sex and the necessities for living as a female much sooner.
