I would assume that it varies quite a bit from department to department. I'm a 1st year now, and while I'm not at all considering a switch, I am probably going to be shifting my secondary field and nobody really minds that. If I did want to change my primary field, it also wouldn't really be an issue; they brought me in to be a PhD student and a scholar of political science, wherever my studies tale me, not just a student of political theory.