If someone leaves a categorical neuro program at the end of the PGY-1 year, then a PGY-2 spot will open up. I've personally known categorical neuro residents switch to psych, anesthesia, path, and even derm after internship year. Spots were filled by IM residents or a recent IM graduate. If you're an IM resident interested in neuro, talk to the neuro PD at your hospital to see if there is an open PGY-2 spot, especially in a categorical program. However, make sure your medicine PD is aware of your interests.