If you mean transforming the male genitals into female ones, it is a matter of debate if that is possible in the ES universe. People can use magic to summon monsters from other dimensions, talk to deities, create undead, heal the worst of wounds and diseases, steal souls and use them for batteries, etc... How difficult is it to turn an innie into an outie compared to all that?
Don't know. I mean, they all seem non sequitur. That is to say, just because great feats are possible in the universe, doesn't mean relatively more minor ones are.
IRL, in the ancient world it was common for male to females to simply cut off their male genitals, often as part of a religious ceremony. They supplemented that by taking what amount to estrogen pills from plants with naturally estrogenic properties, like wild yams. Because these usually double as birth control, they are already known in their societies (the first modern birth control pills were developed from a Native American contraceptive using wild yams). Then it is just a matter of growing your hair long and wearing female attire. In ES, one can easily imagine alchemy supplying the estrogen with no problems at all. I can also picture magical hair removal as well. In fact, I bet that would be a big hit with all women at the magic day spas.
I was aware of the eununchising, but not so much the estrogen-heavy food consumption, although logic would dictate that conclusion. With that said, "one can easily imagine alchemy supplying estrogen with no problems at all," has it been proven that estrogen / testosterone / etc... exists in the ES world? Once again, gender could possibly come from something else entirely different in the universe. Meh, hypotheticals... could go either way.
EDIT: Clarification