Switching programs is not common, or at least not as common as you think.
In addition, you've decided to switch fields before you even start in your current field. I can see how that would be concerning. Let's say that your program that you're starting in 2013 contacted you and tells you that they've had a change of heart, decided that they don't really want you anymore. They know you've matched and so of course they'll take you for a year, but basically at the end of the year they're not going to renew you. You wouldn't be very happy, I expect.
Now imagine that happened to someone last year, and you had heard all about it. Chances are you wouldn't be very excited to go to that program any more.
This is the way PD's will see it. Believe it or not, residents are an investment. We want our PGY-1's to become PGY-2's, and so on. PD's might see you as flaky. Maybe you'll start some new field, then decide the initial field was more to your liking and change back again.
Also, when we see someone change fields over and over, we worry that you're actually failing out of each program. Sometimes it's easier to let someone gracefully resign than to fail them.