I'm also a little confused. Weren't you just in grad school and thinking about going back to finish your pre-med pre-reqs 2 years ago? Are you already considering different fellowships?
I don't imagine its "impossible" but like anything else depends on a few factors:
- the reason you want to switch
- support of your current fellowship director
- being in good standing in your current fellowship
- in training exam scores, residency evaluations, etc.
Then I would imagine the procedure would be to go through the match for your preferred fellowship training or if a position were to open up mid-year you could obtian it through other routes.
I cannot say I am familiar with anyone who has done this except one friend who did change fellowships in October of his first year (new specialty, but went back to the hospital where he did his residency which just happened to have a fellowship opening). Basically, it can happen but depends on a lot of factors.