Jeff realised after checking the range function of the Chronivac that it could transform anyone and everyone in the world. That gave him a really fun idea: he could try changing the genders of EVERYONE. That way no one would find it weird that he'd changed gender, and it might give everyone a whole new perspective on life.
He set the Chronivac to change everyone, and once he hit Apply it reached back into the chromosomes of every human and reversed them. Everyone who had had a Y chromosome now had a second X, and everyone who had had a second X chromosome now had a Y. The switch reverberated out to their present bodies and they started to transform.
Naturally, half the world changed one way and half the world changed another. Everyone who had had a male body had their figure subtly shrunk and turned more curvy and petite, their skin smoothed, softened and made less hairy, their nipples expanded and chests enlarged as they formed mammaries and sensitive breast tissue, and their penises shrunk down into clitorises while their testicles moved inside them and became ovaries linked to a womb and their scrotums receded into their crotches and peeled open into moist vaginal folds.
By contrast, everyone who had had a female body had their figure firmed up and grown with larger muscles, their skin roughened and made more hairy, their breasts shrunk away into flat pectorals with small nipples, and their vaginas sealed up to form sacs which their ovaries moved inside and reshaped into testicles, while their clitorises expanded and thickened into penises.
There were several things that changed along with people's bodies. Naturally everyone who had been pregnant and became male was not pregnant now, but the people who had fucked them were now pregnant with the child, whose sex had also changed. Everybody who had been gay was still homosexual, just attracted to their new gender rather than their old one, while everyone who had been straight was attracted to their old gender, and all trans people were the opposite gender to the one they had been before, so they weren't suddenly cis.
This was kind of a lot to take in for Jeff- or, to give her her new name, Jess. She had really enjoyed the sensation of transforming, undressing to see his body reshape, breasts form at his chest and his penis shrink away into her vagina, and it had felt indescribable how his mind shifted to see herself as female. But realizing how dramatically the world had changed made her wonder about some stuff...