When you start law school, all the administrators and professors say to you, "If you think you know what kind of law you want to practice, you are wrong. You will change your mind."
I came to law school to be a prosecutor. My desire to do so has not diminished at all. What has happened, is that I have realized that you have to leave your options open. That dream job may not be open for you when you graduate. So, you have to decide what you would be comfortable practicing. For me, I know business law would stifle me and kill me with boredom.
Working where I do has taught me that I want to prosecute, but I could easily do defense too. This is, what I believe, a good attorney can do. S/he will be comfortable on either side of the table, willing to deal, willing to enforce the constitution.
I say this to everyone so that you know, that yes, you could change your mind. But you might end up being like me and knowing that criminal law is what I want to do and it has not gone away, it has just broadened to a new understanding.