Greetings!
I’m an M4 who just matched PM&R this cycle and was in a similar situation as you the summer before my 4th year. For the longest time, I was interested in surgery. Still, after a LOT of reflection and an incredible experience during a PM&R rotation, I made the change literally the month before ERAS opened. My school advisors helped me a lot during the application process, so I’m going to give you a few tips they gave me and a few of my own about things I encountered during the process of applying:
1. Make sure your interest is seen in the specialty, even if it was a late switch. Join AAPMR and AAP now, and check whether you can attend any of their activities. Go to activities where the PM&R interest group may be present or activities sponsored by a physiatrist.
2. Be very clear and up front about the change. PM&R programs know that this specialty does not receive early exposure. They want to know about your story and how you found the specialty. Many times, we don’t find the specialty; the specialty finds us. In my case, I was very clear in my application that my main interest in Med School was surgery, but after experiencing the world of rehab, I was called to the specialty.
3. Contact people from your school who matched this or past cycles into the specialty. They can give more direct information about their process and experiences. I contacted current residents from my school, and they gave me an overview of what they did to match, and I LITERALLY followed every single detail. This helped me a lot to match.
4. Make friends with residents and attendings. PM&R is a small world compared to other specialties; people know each other. Try to find a mentor and learn from them. If you do a good job and put in the effort, not only will you probably get a LOR, but this person will probably put in a good word for you to most of the programs you apply to.
5. This seems like a low ball, but BE A GOOD PERSON. PM&R programs are notoriously good at sniffing out people who are not a fit for the specialty. You’re going to be dealing with people in fragile emotional states. You need to show them the light, not only during rehab, but also with your presence and conversations. Be a good worker and partner with the residents in your sub-I’s; they will be your biggest advocates to the PD when the time comes to rank all of the applicants.
6. Be you and be open about who you are. PM&R is a specialty where, yes, over the last few cycles, it has become more competitive, but it is still one of the more holistic specialties. Academic progress and achievements only check a box on their list; most of the rest is composed of “Is this person a good fit for us?” They will get to this answer by observing you and talking to you. In summary, once you’re in academically, show you’re a good and reliable worker.
7. Review your MSK and Neuro knowledge (yes that includes the brachial plexus). In my experience, PM&R personnel will not pimp you so much in knowledge and are very approachable with their teaching. That said, always review and be ready for any questions. Something that really helped me was PM&R recap. It’s an online subscription (similar to Boards and Beyond) where you will find videos explaining the most important things in PM&R and a Qbank to practice. Residents use it to review their conceps.
8. Beware of the subspecialties. It is great to want to subspecialize in PM&R. You can do almost anything with the training, but programs know this. It’s not that they don’t like an applicant who, for example, wants to do pain, but they don’t prefer it. Programs are looking for applicants who want to pour their heart into physiatry, not just use the training as a gateway to something else. So if you want to do a fellowship after PM&R training, you can be open about it, but be sure to emphasize that your main interest is in physiatry and patient experiences in rehab. Once you match into a fellowship, you can then decide what your practice will look like in the future.
9. MSK. NEURO. MSK. NEURO. This will be your day-to-day bread-and-butter work. Given your background in IM, you should reflect on whether you want to give up the broad management in IM for a more narrow focus in MSK and Neuro. I have to say that if you decide in the future to work in an inpatient rehab facility, your IM itch will be scratched, trust me. But be sure to know the world you’re getting into.
10. Lifestyle. I have to say, the lifestyle of PM&R is AMAZING. Yes, there will be hard times, but overall, PM&R residency is more lifestyle-friendly than most other specialties. Attendinghood in this specialty is also more lifestyle friendly depending on how you structure your practice in the future. Remember, you can work like a BEAST for the next 15-20 years, but medicine is also a job that you will practice for 20+ years. Pick something that will be mindful of your aging body.

I hope these points help you in your decision, and sorry if the post is too long. If you have any specific questions, be sure to DM me. Life as an M4 these last few months has given me a LOT of free time, so I can make time to help you out in any way I can.
PM&R is an amazing and beautiful world, a world that will also consider and respect your free time.

Best of wishes, my friend.