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Good question! Short answer: generally you refer to them as if they have always publicly identified with their current gender. Longer answer: The overarching idea is to respect the person's wishes and stated identity. In most cases, and certainly in Rebecca Heineman's case from what I've read, she always identified as female even when she was known as Bill Heineman. So out of respect we would typically refer to her a…
Thanks for the answer ! I'm asking because the change of gender also associates with a change of name, and it kind of seems like rewriting History if ex-colleagues talk about X-woman as X-woman in the past when she was not yet X-woman but Y-guy instead. How about legal documents ? How do they handle the change of gender ? Do they do it like "X-person, Born Y-person" ?
Here in the UK, you can go before a sort of judicial committee to do it, the Gender Recognition Panel; costs like £140ish. It's kind of a pain; you need to be at least 2 years in full-time, it does help if you've had SRS or at the very least are going to (and they're going to want medical reports), and you'll need two medical practitioners (one of which must be recognised in a list as a psychiatrist or psychologist specialising in gender dysphoria) to testify that yes, you are trans, yes, really you're not going to change your mind, and this is what you've done/are going to do about it. If you pass (and it's a little bizarre and intimidating that they're judging this), you get a new short birth certificate (and can apply for a long one, and thereafter a passport under your new official identity); you then have two proper birth certificates, the historical document you were born with, and a brand new one listing only your target gender (and name). The fact that you have done all this is confidential (unless you tell someone, obviously).
Your mileage will vary, naturally.