Matching into FM as a "placeholder" was not a good idea. What's done is done.
There's some chance that you'll start FM training and realize you really like it also, and that it's better than EM. So, I'd go into your PGY-1 with a positive attitude if possible.
But let's assume you're 100% committed to EM. First, you need to assess whether you're competitive for EM. If you have poor exam scores, failures, problems, etc, then EM may simply be out of reach and although you can try, you should plan this as a "hail mary pass" with a clear backup plan, which might be your FM program.
If you're competitive, then you'll want to get LOR's for EM (called "SLOE's"), at least that's what's needed in an ACGME program. Because you may have little to no control of your schedule as an intern, you should get everything you need while you're still a student. Then, if you can get some ED experience as an intern, that's even better.
When to talk to your PD doesn't have a single right answer. You could start talking to them now. There's some chance that they will offer to void your match/contract -- but you have to decide whether that's what you want or not. In any case, you'll be applying in September again for spots, and will need to interview. Remember that being a resident is a job, you get paid to do work. Most employers will not be very accommodating when you say you need time off to find another job.
You're also going to run into a problem with timing of next year's match. The NRMP match happens mid March. If I have a PGY-1 who is leaving, I want that spot filled long before then. Therefore, you may be forced to give up your slot before you know whether you've matched somewhere else. Often people say here that their plan is to "try to match, and if not just continue with their current training", but that may not be an option. My personal style is I have the person apply to the field of interest. Once they see how many interviews they have, they must make a choice.
There is also the problem of GME funding. This is a complex issue, but each resident receives full funding for a fixed amount of time. Once you start FM training, that will be 3 years for you. This might limit which programs would consider you, as any training beyond that would be at a decreased funding level.
Last, it may be possible to work in an ED after FM training. There are 1 year EM fellowships for FM residents. However these are being phased out, and some ED's only consider fully ED trained docs.
