Jeff felt the changes start to take effect.
Now the Chronivac does take the DNA and the percentages but they aren't always what you expect. Rather than taking the muscle of the jock and the brain of the nerd it reversed it and taking those changes and combining them it produces a hybrid with those perimeters. It would have been too easy to just give Jeff the body of Nathaniel who was a scrawny little nerd and even though Tyrone wasn't the smartest guy in the world he wasn't riding the little bus either.
When the changes were made it took into account the history of the person being changed and that was where Jeff got into trouble. The intelligence of Tyrone was dropped severely and he wasn't interested in learning partly because the change made Jeff dyslexic. It made it hard for him to learn so rather than learn he played video games. Having never worked out in his life the Nathaniel part of Jeff showed a huge change in his body. Huge was an understatement. He was almost round. Nathaniel wasn't tall but the adjustment that the Chronovac made was that he was now 5'3" tall and almost 400 pounds. If there was a real life version of Cartman this would have been it.
Jeff felt the little bit of him slipping away and replaced with the new version.
Jeff waddled over to his XBox and sat down and started to play. He pulled up the Chronivac once and got confused at the choices and immediately shut it down and tossed it in the trash. Jeff remained this fat dullard for the rest of his life being used and abused physically and sexually but he didn't know enough about what was happening to care.
The next person that got the Chronivac didn't fare as well as Jeff did.