Generative AI may change how firms define occupations. We study this process in software development, where large language models overlap with tasks commonly assigned to junior workers. Using the near-universe U.S. online vacancy data from Lightcast, we examine how the public release of ChatGPT changed entry-level software hiring standards. Event-study and difference-in-differences estimates show a 14–15 percent relative decline in junior versus senior software developer vacancies, larger than in related technical occupations and absent in mechanical engineering. A shift-share decomposition shows that rising experience requirements were driven primarily by employers asking for more experience within the same job titles, not by asking for a different composition of titles. Remaining junior vacancies shifted toward problem solving, communication, and attention to detail, not AI-specific skills. The results show how generative AI redefines entry-level work by raising the bar for what counts as a qualified junior hire.