I do know of a person who started in OB, things didn't work out, and then they were able to secure a spot as a PGY-1 in EM the following year. I don't have the full story, but I seem to recall the program this person was leaving was very cool about it, and supportive of the effort (person was allowed out of the contract).
Not a lot to work with I know, but in short, it's not unheard-of, and losing a year can be worth it in the right situation. Last I heard, this person was way happier in EM than they had been in OB. Good luck to you!