I'm applying to Pain this year from EM. Graduated residency in June so I just started working as an attending. Not sure how the Match will turn out, but things have gone better from the interview side of things than I anticipated. Wish me luck!
Anyway, the most important thing is doing at least one Pain rotation at an academic center with a Pain fellowship. Doing a rotation or two will help you to learn more about the outpatient treatment of chronic pain, medication management, fluoro-guided procedures, etc. It also allows you to know if you actually like it. Pain is extremely different from EM as I'm sure you know. Finally, a rotation will allow you to get LOR from academic Pain faculty. This will help your application immensely. If you decide that you definitely want to do Pain, you can do more rotations to get more experience and additional LOR. Otherwise, just make sure that you understand the application timeline and get stuff ready for ERAS, like asking for LOR early and having a good personal statement ready for when ERAS opens up. If you want to go directly to fellowship from residency, you need to apply as a PGY-2 (or PGY-3 in a 4 year program).
Agree with reaching out to Birdstrike. He's super helpful.