We have a faculty member who asks: what do you envision will be the title of your dissertation? I think it's a ridiculous question and I told him so a couple of years ago ... His take was that the questions (or answers) aren't as important as how people answer them. In other words, he's perfectly aware that most people interviewing aren't going to know exactly what they want to do their dissertation on yet. .. Actually, the question still annoys me but I was able to answer it and remember kind of having a bit of fun with it, even though it's ridiculous (and it turns out that my dissertation is actually going to be pretty darned close to what I said it would be back then ...)
So don't worry so much about being held to what you say at the interview about future plans and future interests. Most of the people interviewing you know that you will grow, change, develop additional interests, become interested in different aspects of a broad area that interests you now, etc.