I think there are three things to keep in mind:
- What is realistic for the POV character. Are these flashbacks from the trans character's POV? Another character's? If the latter, what is the dynamic like? Someone who's always known the trans character as a woman will likely use female pronouns. A relative who's had a difficult time not seeing the character as a man might have a different instinct.
- What is the framing of the flashback? Are we in the POV character's present-day head while they think back, or are these scenes that are actually taking place during an earlier time in the character's life? How she thinks of herself now, even in the context of memory, could be different than how she thought of herself years ago. Same thing with how other people think of her.
The "girl" who made me realize that I was bisexual is now an out trans man. I think of him as a guy and would use male pronouns when talking about him, but if you were in the head of my 14-year-old self, I obviously thought of him as a girl because I didn't know he was trans yet. My own perception of my gender and sexuality have also changed since I've realized I'm non-binary.
- That said, the generally appropriate thing is to not change which pronouns you use for someone based only on whether you're talking about them pre- or post-transition. Some trans people, like Maryn's daughter, find it very offensive. For others, it can seem kind of...out of touch. There's this old-fashioned perception that transgender people change their gender when they transition, but that view no longer resonates with a lot of trans people, if it ever did. A lot of people feel that they have been their true gender all their lives, and even if they don't feel that way, most people don't have a life that's cleanly divided between pre- and post-transition. Transition is a long process, and some people never medically or legally transition. Transitioning doesn't "set" a person's gender identity.
But that doesn't mean that there aren't any instances where it makes sense to use different pronouns, particularly if you're in the POV of a character who reasonably would do so. It's just something to be aware of, and not something to default to.
