As in, a new birth certificate that states the sex that an adult wishes to live their life as. Saying that Michael is now Michelle is a legal fiction; it doesn't change the underlying biology, and everyone, including M, should realise this. It shouldn't let M think that M is now a woman in the sense that 99.9% of us are women, which is what TRAs imply. Some people have chanted "Trans women are women" so often that they seem to believe that trans women are on the verge of menstruating and bearing babies, which is loonytunes, and IMO leads some mentally unstable or intellectually wobbly people into dark places. The phrase "legal fiction" provides sunshine.
I see a legal fiction as a polite strategy to let individuals get on with their own thing. It helps society run smoothly. Adoption is the classic legal fiction. A more peculiar one is MPs resigning by "taking the Chiltern Hundreds": Wikipedia says legal fictions are a very English thing (although it also gives Jewish and Hindu examples).
A legal fiction is a way of suspending disbelief. You don't have to really ultimately believe something in every sense, if it is convenient to act as if you do believe it. I want a baby and enter into an agreement with Sarah Surrogate. She gives birth to baby Jane. Jane is legally Jane Tiggywinkle, and I have all rights and responsibilities of a parent. However, I am sane, and do not believe I need postnatal check-ups of my reproductive organs (though come to think of it I might appreciate support to head off postnatal depression). Sarah Surrogate is also sane, knows full well that she is not the mother, but avails herself of relevant services. In later life, if health providers ask if I am a mother, I will clarify their question - yes I have a child, no I have never been pregnant. Motherhood in this case is a legal fiction (and much more, of course) but not a biological state.
I don't see "fiction" as a disparaging word. The novelist's craft is not lies, but getting at a deeper human truth. A metaphor is true when it is useful, and not otherwise. So Michelle is a woman, as a legal fiction. I don't see that as a negative thing, though I am sure TRAs will despise it, and it may hurt the feelings of some non-activist trans people.
And "legal" of course refers to how wider society is required to treat the person. It acknowledges that they should be treated as belonging to a certain category, but it opens the door for consideration that that category is not always the correct one. It allows room for lawyers (local authorities, government departments, committees doling out women's prizes, etc.) to discuss exceptions.
Can we not resurrect this language, as one of respectful recognition of difference?