You can change your field of interest easily; no one will stop you. However, there are advantages to not switching: profs who read the applications will keep in mind who looks good in their area of interest, and may pass word on to their colleagues: "hey, John Smith is interested in your field and looks good". This makes it easier / quicker to get into a good group. When you switch fields, you are back to square one since no one will know you in that area.
So, I suggest you fill in the application with the thing you actually want to do. Then, if you change your mind, just switch.
Best of Luck!