What is supported in the scientific literature is that early - onset trans people both ftm and mtf are more likely to be gay-identified prior to transition, and keep primary attraction so more likely to be hetero identified after transition. Conversely, late-onset trans people are more likely to be hetero identified before, gay after.
That holds up pretty well in my experience, but certainly not 100%.
There are girls (and I assume guys tho I don't know any) who wind up relatively hetero-identified before and after, so yes what you ask does happen. I assume there are also some who find they're gay or lesbian both before and after.
Anecdotally, this has been a surprising change for the people involved. It's something I've wondered about also and would consider a negative outcome.
While nominally bisexual, I'm quite mistrustful of men and also quite sexist toward them. I very definitely do not want to change my orientation and yet knowing others have experienced that leads me to be prepared for the possibility.
Now that I'm on a viable path to transition, I'm also able to imagine sex with a neovagina - before I decided and then started hrt, imaging that was just too painful and so I've contented myself with occasional penetrative anal from my GF and while the opportunity hasn't presented itself I've been open to trying that with a guy (anecdotally the real thing has more pleasure potential than a strap on).
In all of my sexual experience it's been extremely rare that I can simply feel feminine in the moment of orgasm, think maybe twice in 20 years of knowing I'm trans. I'm not good at suspension of disbelief.
That's changed a lot under HRT, I don't even think that's due to the estrogen, rather seeing a path forward it no longer hurts to imagine vaginal sex. That's included more ideation around sex with men. I can't double blind this, knowing it's an issue for me, the thoughts could as well be need to explore where I'll wind up as anything due to the HRT itself.
So I may be shifting that way but for me I strongly suspect it's more in my head than in the hormones.