When you look at who you're actually attracted to with your current body, sexual orientation can change. But does that mean that it has actually changed? Not necessarily.
Let me explain...
When I first started HRT, I suddenly found myself becoming attracted to men. And at the time, I thought that it was my sexual orientation "changing." But really, now that I've had time to think about it, it actually didn't.
My sexual orientation was always as a bisexual woman. That's the key, though... I had to be a woman first. When I still had a male body, I was not attracted to men, because there was nothing that I could do with a man that would even remotely resemble what I was attracted to. Because, well, stroking a man wasn't what I was imagining, it was the being stroked while I had a soft feminine body, and being penetrated through a vagina, that I was attracted to. Whereas with a woman, the rough equivalent of lesbian sex was indeed possible, because as long as you're the one taking the active role, you can do the same things to a woman that a lesbian would do even though you have a body that's technically male. I was with a girl, and as long as the attention remained on her, I was happy. The problem is, though, she wasn't imagining lesbian sex, she was imagining heterosexual sex. So when the attention turned back to me, and my male body suddenly became involved in it, I felt uncomfortable, and it felt weird, and I just didn't find it attractive.
So in my case, I thought that I was exclusively attracted to women, and I thought that I was heterosexual for years. But really, it wasn't normal heterosexual attraction. It was just the closest thing that I could get to a normal sexual relationship that somewhat resembled my internal sexual orientation.
Now that my body is becoming female, now that the possibility does indeed exist of having a sexual relationship with a man that actually fits that fantasy, I've found myself becoming attracted to men. Suddenly, their strong holding arms seem attractive, and the contrast between my soft feminine body and their hard muscular body seems much more attractive. And really, again, that's the way that it's always been. Pre-transition, I was just as excited by the thoughts of both being with a man as a woman, and being in a lesbian relationship as a woman. I just couldn't fully express those desires until I myself had a female body, and thus my outlets actually matched my fantasies instead of just being pale imitations where I had to ignore the physical reality of my own body in order to get into it.
So to answer the question, no, your sexual orientation probably won't "change." But you may discover what it really was all along. As you feminize, as you find your fantasy-self getting closer and closer to your real self, you'll find a lot of those idealized images in your head slowly manifesting themselves in your real attractions toward people. And thus it's quite possible that it may have the appearance of "changing" even though they were actually always there, and always the same.
