"You know what Jeff? You should let me use it instead. Either you're pranking me, in which case you'll have even more reason to mock me, or you truly believe it will work and, since you asked me what to change, it would be better if I was directly using it. Don't you think? And no peaking! If by miracle it works, I want the changes to be a surprise to you."
As Jeff got back to his bed and turned his back to the Chronivac, Jenny got to work. She had always liked those stories in which some kind of parasite, be it an E.T., a gost/spirit/demon of some kind or anything in between, took over the body of a poor human, making said victim be nothing else than a spectator of their own life with no control left of their body. A small part of her even wished she could be the one being taken over by such a parasite.
So she programed the Chronivac to alter humanity so 0,0001% at most turn into parasites between 16 and 18 and need to take over a non-parasitic human's body. Usually, those parasites take over a body of the same gender than their own, but not Jeff. Jenny and him were on a date when he changed and, since her parents never approved of him, they had gone to a place where they could be all alone, and Jeff hadn't had a choice but to take over his girlfriend body, unknowingly realizing one of her dreams.
Parasitics humans are a mostly accepted part of society and, while most don't want to end up as their host, there is a small community of people that want it, like Jenny.
Once someone turned into a parasite and took over an host body, they're given a new ID card that indicate their condition as host & parasite, the name of the host and the name of the parasite, but all that is only for administratives purposes. Parasitics humans prefer to be called by their host name and treated as if they actually were said host.
Once parasited the body develop a second voice on top of the host's original one, which is only used in the extremely rare case the parasite need to speak specifically as themselves. In the case of Jeff, since he is male, it make an Adam's apple temporarily appear on Jenny's neck. (In the case of a female parasite with a male host, when speaking as herself she would make her host's Adam's apple temporarily disappear.)
Since Jenny's parents never liked Jeff, after he took over her body, her parents became violent with him. Thankfully it was rapidly discovered by Child Protective Services and the custody of Jeff/Jenny was rapidly given to Jeff's parents. (Separating a host from his parasite can't be done without killing the host. The parasite on the other hand, has a good chance of surviving as long as they find another host.)
As Jeff's host, Jenny didn't have any secret for Jeff, and couldn't have any. He know all her thoughts and have access to all her memories and knowledge. Not only that, but because of his parasitic nature he usually imitated how Jenny would act, and did it so perfectly her friends often forgot Jeff was the one in control.
While they were still too young to think on it, as a male parasite Jeff would still be able to have children with Jenny when the time came, despite her being his host. (He could also impregnate someone else if he ended up falling in love with someone else, but since she was already programming the Chronivac Jenny ensured it wouldn't be the case, and they would be each others only lovers.)
Of course as a parasite, while Jeff still identified as male, he didn't mind that since his host was a girl, most people treated him as one. In fact, the very fact he took over a female body made him need to be misgendered by others, need peoples to think he's really a girl named Jenny O'Brian rather than a male parasite named Jeff controlling her body, and act accordingly.
She validated and immediately lost control of her body.
At first Jeff didn't understand why he suddenly found himself in front of the Chronivac, or in his girlfriend's body, but in less than a second that changed thanks to his intimate access to everything Jenny his new nature as her parasite gave him.
Looking around his room, he saw it didn't look like a boy's room anymore. If anything, it looked like Jenny was the one to decorate it, with the intention to make it her room. Because of his new nature, it made him smile.
Walking to the nearby mirror, he admired the body he had snatched in this new reality, the body that used to be his girlfriend's but was his now, and slowly undress, doing a striptease of himself for himself, teasing himself with the tantalizing flesh of his very feminine body.
Once nude, he cupped his breasts with his hands. They were nice C-cups.
Having him strut around in her body was very arousing to Jenny, to the point that, if it was still her body, it's pussy would be dripping with juice. Looking coyly at the mirror over his shoulder. "Oh, you naughty girl," he said in Jenny's voice.
As parasite, he had absolute control over his body's arousal, letting him decide when it was aroused and how much. It wouldn't do to let such a thing distract him, after all. He know those of his fellow parasites who end up with an unwilling host often punish them by making their body extremely aroused but denied them any kind of release, or cut them from any feeling when said release came. But Jenny was far from unwilling, she was even the only host he ever heard of that reveled in being a prisoner in her own body. So to recompense her (and himself, lets be honest) he got on his bed, let his body be aroused and started masturbating.
He did it multiple time, first with his fingers, then with each of the sex-toys from the extensive collection he bought specifically to please his girlfriend once she became his host. Naturally, he reacted to those stimuli exactly like Jenny would have, to the point she temporarily forgot she wasn't in control of her body anymore.
After reaching their last climax, Jeff took his latest toy out of their clit and they both fell asleep in bliss.