Hopefully your desire to switch from FM to IM is not spuriously fostered by your notion of how the two specialty's inpatient teams were run at your specific hospital.
In case it was, do understand that every place is different and each hospital has its own way of managing how it runs inpatient. If your concern is feeling that you will be lambasted with problem patients and unchecked censuses, then your energies may be better focused on finding a position that offers what you look for.
In terms of the inpatient/hospitalist field, there is a lot of overlap in the two specialties and it's hard to imagine one being unable to find a position to one's liking regardless of the field you are coming from.
However if your desire to switch is solely to subspecialize, it might be worth some further insight into seeing what you believe the subspecialty field can offer you and why you believe you cannot find it in your current FM specialty or any of FM's various fellowships.
It has nothing to do with a specific institution, hospital, or service. I was trying to avoid details, but I'm not a US citizen or green card holder. I now live and practice in a country that prides itself on having a world-class healthcare system (if you keep reading, you'll see why I mention this). Here, the fields of FM and IM are very different. Quite simply, as a GP (general practitioner), I cannot practice the medicine to the level I was trained and remain financially viable. I say "GP" with gritted teeth because the system here clumps me into that category - FM does not exist here as a specialty like it does in the US. Traditionally, GPs are MDs that finished medical school and then completed a single year of random rotating internships without any formalized/structured FM training. What this means is that most of my colleagues are clueless or woefully behind (including those who are training the next generation of physicians) when it comes to practicing evidence-based medicine. Don't get me wrong - many of them have such vast experience and knowledge that it blows me away and some of them are fantastic physicians. But, the majority are not, and unfortunately, the system is set up to reward only quantity of care. As an example of this, I routinely see patients from other GPs who have been prescribed narcotics for years to treat their migraines. Most patients on opiates that I have encountered also do not realize that they are on an opiate/narcotic.
Quite simply, I am supposed to be a glorified doorman who is supposed to simply triage and refer people on to a specialist because the five- to six-minute appointments I'm supposed to strive towards seeing people in are not sufficient for digging into a problem, working it up, and treating it. I do spend the extra time to practice proper medicine, but it means that I need to look for other sources of income to remain financially viable. Sadly, I also see many of my peers who trained in the US falling into the habit of not practicing proper medicine (like prescribing medications and ordering tests in lieu of doing a proper history and exam) in order to earn more. Am I being judgmental? Definitely. But, there are some things I refuse to cut corners on and quality of care is one of those things.
I realize working in the US as FM is an option (for most locations, it would be much more money for less work), though tricky because I would require a visa. As of late, I have been considering it seriously, but worry that I would not be able to adapt back to the US system with all its nuances - documentation, CYA, insurance, ICD10, and so forth.
To answer your other question, I may also be interested in subspecializing.