Sometimes you just don’t know if you’re going to like something until you try it.
And you don’t have the opportunity to spend as much time as you may need in each field. Or maybe you've simply changed as a person. It is reassuring to know that in those situations, it is okay to change your mind.
Why would you change residencies? It could be that you were not sure what you wanted to pursue, or that you matched into your second choice field and aren’t ready to give up your desire to practice in your first-choice specialty. It is okay to go ahead and re-apply to your first choice.
The most important point is that it is okay to change. People change, and they can change where they are training and what they are training in. But if you are going to change your mind, do it earlier in your career, and not too often.