My orientation didn't change (I was bi before and bi after), but my experience of sexual attraction changed dramatically, I went from not being capable of much attraction to men to very obvious / overt attraction to men.
I knew I was theoretically bi and not fully straight before I transitioned, but there was never a time that I saw a boy or man and felt sexually attracted to them. I never had a crush on a boy or man, etc. and I would never actually want to have sex with men, etc. - so I was a 1 on the Kinsey scale (predominantly attracted to the opposite sex, women, only incidentally attracted to men) before I transitioned (this assumes I was "heterosexual" by being attracted to women, though my experience before I transitioned was as though I was a kind of lesbian, so ... I don't know, I just call it opposite sex because that's what everyone else saw - I don't mean it to represent more than that).
After many months on estrogen, I started to be able to see men in a completely different way, and could feel sexual attraction to men similarly to the way I could feel attraction to random women. I remember in particular walking onto a plane and walking down the aisle and looking up and seeing an attractive man and feeling that flush of sexual feeling / attraction and realizing: oh, that's never happened before.
I don't think this was a psychological shift due to being more open due to transitioning or anything, this was a biological change due to the hormones. I had lived as a woman for months before I started estrogen, and I experienced plenty of psychological changes and opening up during that time, but increased attraction to men didn't happen until many months on estrogen.
Not everyone experiences this, by the way, and it's impossible to know for sure how you will feel - but my understanding is the hormones don't change your sexual orientation, even if they might balance or alter sexual attraction. I'm much more bisexual now, and my attraction to women also diminished over time such that I have more equal physical attraction to both sexes. Regardless, I only have romantic interest in women and would never date a man, lol.
Either way, you can't control whether you start to feel physical attraction to men, but you can control staying with your partner, and you are already capable of being attracted to other people than your partner and it will be no different after HRT. I've never heard of someone's sexuality entirely reversing, i.e. starting off attracted to women and then becoming exclusively attracted to men - I'm pretty confident that's not possible, even if it is possible for incidentally bisexual women like me to become ... more bisexual.