Sequential hermaphroditism is when a species starts out as one gender and then changes over the course of their lives. Most animals who experience this are very simple and have very low degrees of sexual dimorphism, and humans are one of the most sexually dimorphic species on Earth.

However, could evolution or genetic alteration, give humans the ability to do this as well?

This is incredibly speculative, but I am working on a science fiction project and wanted some ideas from real science

However, I am uncertain if the transformation itself would prove fatal for the human as it might be excruciatingly painful as the spine and bones would have to change, and there is no biological mechanism to transform genitalia from one form to the other. Breasts should be doable. They are just fat deposits. Also, you would need a lot of metabolic energy (presumably) to make the change, and wouldn't the transformation give off a lot of waste heat?

My questions are (besides is it possible?)

What event(s) might trigger the change?

Would the subject grow or shrink in height or stay the same size?

Would the transition be gradual or would it take place over seconds or minutes?

If the brain changes, would this effect the hosts memories or sexual orientation?

How many changes could occur over the course of one lifetime (assuming the average lifespan stays at 71-75)

Again, this is very speculative but I'm looking for ideas from real-world science to make some educated guesses.

Humans are quite different from those species (mainly fish or snails/slugs) which do that. Our X and Y chromosomes are quite different, which doesn't allow changing from small to large gametes or v-v in an organism. And we have other complexities in our morphology (and its sexual differentiation) that don't make such a switch feasible. And bone structure won't change after puberty is done - you might look up growth plates to understand why that is. Even very minor gene alterations that affect morphology and sex can cause severe deformities and pathologies. Turning us into slugs would not go well. (Ethical issues aside)

1 hour ago, Maximum7 said:

However, could evolution or genetic alteration, give humans the ability to do this as well?

Would that not take all the fun out of sex ?
( you're not 'incel', are you ? )

Edited 3 hours ago3 hr by MigL

1 hour ago, Maximum7 said:

Most animals who experience this are very simple and have very low degrees of sexual dimorphism, and humans are one of the most sexually dimorphic species on Earth.

That is not very accurate on a couple of accounts. First, the mere mechanism of hermaphroditism makes the concept of sexual dimorphism not terribly useful, so this claim in itself is tricky. The second part is that humans are by any measure on the extremes of sexual dimorphism. Anglerfish are an example I like to use, where the male essentially becomes an appendage to the female. However, even among apes, humans are among those with the lowest degree of sexual dimorphism, for example if we just go from average build.

And ultimately all of the question are, of course fantasy. Transition has been documented in fish, for example, but you won't find them in humans. That being said, there are intersex cases in humans which will tell you a bit about sexual organ development and how intermediary forms could look like. But you are confusing a lot of primary sexual characteristics with things gender (not sex)-associated parameters. I think it boils down to a very specific view of sexual dimorphism you might have, that simply does not translate well in cases of hermaphroditism (or intersex) where those categories mostly break down. Similar situation with reproductive species definition in ring-species.