Jeff was insanely horny. Even more so than normal! Here he had a machine that could irrevocably Chang his life forever if he used the "permanent and irreversible" setting.
He was becoming so horny, his hand was shaking. He started setting it all up to match his phantasies. He set it up to exchange places with a donkey already in the field, and set it where he would transfer his current horniness over to his new body he would take over.
He set the intelligence of the donkey when it takes his human body to be Jeff's current intelligence. Give him his current personality, and take his current personality and human intelligence with him, while adding the donkey's current instincts to take over. That option was meant for his sex drive any time a Jenny in heat was about. That's what it meant to Jeff's mind. But in reality, it would force him to put his head down to the ground and eat grass, and to drink out of the same watering trough as all the other farm animals in his new home, in addition to his sex drive to fuck other donkeys. And much to his later horror, male donkey as well, sticking it in their buttholes and making his penis even smellier.
His heart was racing, and he began sweating when he selected the "permanent" and "instantly" options. He knew there would never be any going back. He also knew he would really, really regret thus, but his overwhelming desire to jack off with his current state of mind forced his fingers into the mouse. He dragged the cursor over to the "apply" button. Pushed down on the left mouse button and held it there for a minute as he worked his human dick to the point where pre-cum was leaking out. The released the button!