Has anybody done that?  Transferring to a different specialty?  How did you explain it to your PD? 
The reason why I am asking is that I really want to do EM.  I am an IMG, not US citizen, applying this year for the first time.  Despite my excellent scores and strong LORs, only 1 interview for EM so far.  On the other hand, I have a handful of interviews for IM.  I have spoken to several EM program PDs, and the advice was always the same: start your residency in IM and transfer after one year.  Well, now how do I explain this to the IM program PD?  I assume I will need his or her reference letter for the transfer, right?  I can imagine how thrilled he could be about me leaving...  Moreover, if I do not succeed next year getting into EM, I would still like to finish the IM residency (I like IM a lot, but EM is my preference).  So what is the strategy here?  Tell, or not to tell?  And if tell, when?
Any advice would be greatly appreciated!