I’m not sure what you mean by “100% conversion”, but no - there are developmental pathways that are irreversible, e.g. the fetal development of the genitals are not going to change by taking hormones later, and many changes during puberty are irreversible (like changes to the skeleton - length of bones, changes to the skull, angle of the hips, etc.).

In trans women, undergoing male puberty causes irreversible changes to the thickness of the vocal folds, and estrogen does not change or fix the voice (but for trans men, starting testosterone will cause the vocal folds to thicken, permanently masculinizing the voice).

Preventing the wrong puberty helps avoid a lot of distress for trans people, which is why it is so important those interventions are available to trans youth. It is no different than if a cis person were administered the wrong hormones and forced to live as the wrong gender - it causes distress and significantly increases likelihood of suicide.

But even for those who avoid the wrong puberty, there are sex differences that developed as a fetus that would require surgery to fix.

EDIT: in case it’s not clear, there is a huge variety in the ways bodies develop even just when looking at non-trans people, and so even though my body underwent male puberty and that resulted in permanent changes to my rib cages, skull, hip width, etc. - my body still more or less overlaps with other female bodies enough that nobody can really tell the difference.

Male and female genitals are actually homologous meaning they have the same structures - both men and women have a phallus, e.g. a penis is a large, spongy clit and women have a “prostate” (Skene’s gland) that like in men produces ejaculate, etc.

So with surgery, my genitals were able to largely be corrected and the various structures were simply altered to be like they would have been (scrotum is a fused labia, and with surgery is fashioned into labia).

So even my genitals would be “female” to most people (not that most people would see my vulva anyway).

My breasts have developed naturally through hormones alone, and at this point I have no complaints about size, and they certainly are large enough that nobody questions that I’m a woman.

I’m beginning to lose sight of in what medical or biologically significant ways I’m not “female” at this point, even if there are still plenty of residual signs of my initial male puberty I’m very aware of, nobody else seems to notice them, and more importantly, my history as a “male” is no longer relevant to my physical health - the current evidence seems to support what I experience: my body is a female body.

This is not something I thought would ever happen, I didn’t think transition really makes you female or changes your sex, I always thought of it as more metaphorical or cosmetic. I underestimated the role hormones play in the body’s functioning and the way hormones seem to play a primary role in biological differences between men and women.