I'm just an MS4 but here is what I think:
Its not uncommon for people to switch specialties during their residency. I think it is fairly easy to switch from IM to Neuro: A good number of people who get accepted to Neuro residencies realize during their prelim medicine year that they would rather do IM. This leaves a number of PGY2/Neuro1 spots open and program directors would be thrilled to fill those spots with someone who has already done a PGY1 year in medicine. Also, Neuro requires a prelim year so your current PGY1 year will count toward that requirement and this current year will not have been wasted.
First, figure out if you really do want to leave medicine and if you think Neuro would be one of your top choices if you do. Get advice from current residents in IM, including 3rd years. When you feel more confident in your decision to leave, then speak with the chief residents, program director, and/or chairman of IM.
If there are any Neuro attendings who cover at your hospital, talk to them. Also talk to the neuro PD at any hospitals with a neuro program that are affiliated with your hospital. See if your hospital will work out for you to do a month of neuro at one of their affiliated institutions.
I think there is a website where people post requests to swap residency positions with other people, so you can look there. Contact the AAMC, ACGME, or whatever the appropriate group is that handles these things and ask for their advice and find out what the rules and regulations are behind switching. You can also start emailing Neuro residencies throughout the country expressing your interest in starting a PGY2 year there next year and they will likely keep you in mind if they get any drop outs.
I can't guarantee you will get into your favorite program or be able to stay in a certain geographical area but I think your odds of switching to Neuro are very good. However, if you wanted to do something else that doesn't require a preliminary med year I think it would be harder because you would probably have to start from scratch as a first year and apply through ERAS.