It's true that if everyone became a vegetarian, some farmers and workers would have to find new jobs. But factory farms employ far fewer people than small farms, and slaughterhouse jobs are traumatic and debilitating. If everyone switched to a plant-based diet, there would be thousands of new jobs created on farms growing fruits and vegetables and in companies making plant-based products.
Keep in mind that there's nothing new or unusual about the process of an entire industry disappearing. For example, people who worked in the horse carriage industry needed to find new jobs after cars were invented.