Autotransfiguration

Shape-changing is broady considered to be an impossible form of magic - something that belongs in myths, fairy stories, and fantasy. Some evidence suggets that shape-shifting may have been possible in past Ages, when the few who had magic were akin to gods, but the the Fifth Age is characterised by widespread but lesser magical ability.
The currently understood laws of magic hold that, in order to change Reality, one must be able to conceive of a version of Reality where the desired outcome is already possible. Shape-changing is impossible, therefore, either because the practitioner is constrained by a consensus Reality, unable to concieve the radical change in their own anatomy, and likewise constrained by their peers being unable to see them as anything other than human, or because the mind refuse to conceive of a total shift into animal (or inanimate) form because it will be unable to shift back.
Some cultures use masking or guising to assume an animal persona for the purposes of a ritual or event, using the mask and costume to facilitate an altered mindscape that allows them to blur the lines between animal and human forms.
It was with this in mind that the great wizards of the Fifth Age began pursuing spells and rituals that would allow a person to change their sex. Transsexuality has long been associated with heightened magical powers, so it is unsuprising that a great deal of thought has been given to creating permanent physical change.
In Erwān magical tradition, autotransfiguration is a multi-stage ritual to bring the various souls into alignment. By the time the ðidngi (individual undergoing the ritual) opts to undergo it, many of the early stages have already been completed. Many ðid'é are comfortable without the final integration (Body II) but, if this is desired, it is necessary to complete all previous stages for the change to be permanent.

Soul integration

Imagination

The first step in autotransfiguration is the integration of the self. The ðidngi chooses with a new name and identity, which they shape by inhabiting it in Dream. This is a private process which can be done repeatedly until the ðidngi is satisfied with who they will become. Many ðid'é write letters to themself or a penpal, or keep a journal under their acquired identity. This vital early stage allows the ðidngi to shape their identity without judgement or commitment.

Community

Integrating the community soul is the process of changing the awareness of people around the ðidngi. The commitment begins with the ðidngi announcing to their community that they have adopted a new name and social role. In smaller communities, this may take the form of a special party or social event. In larger communities or cities, a letter may be published in the Announcements column in the newspaper, pinned to the bullitein-board their community or religious centre, or circulated in a trade or community publication. This coincides with the ðidngi adopting the costume and role of their known sex. In the case of mature ðidngi, they may also undergo a coming-of-age ritual that reflects their known sex.

Physicality

Often concurrent with the Community stage. This is a slow stage wherein the body's natural endoicine profile is transfigured from testosterone to oestrogen, or vice-versa. This is a largely irreversible step. Early in this stage, when the details of the transfiguration are still being evaluated - or in situations where the transfiguration isn't intended to be liflong, an elder isn't available to guide the ritual, or where the ðidngi is a minor - the ðidngi drinks a daily potion called a potency. Potencies are made by apothecaries or pharmacists, and are used until the desired physical changes have been achieved.

Heritage

Often beginning early in the process, preceding even the integration of Imagination, and continuing throughout, integrating the Heritable soul requires the ðidngi to weave themself into a lineage that connects them to their physical community, and to historic and contemporary transsexuals. Understanding this history is necessary because the elders guiding the ritual have been through it themselves, and the ðidngi must be prepared to help future generations in their turn.

Memory

The permanence of the transfiguration requires the ðidngi to fully integrate their identity, surrendering their past claims to their birth sex. This stage is process of recontextualising their memories to reflect their known sex. Ðidngi who undergo the ritual befor reaching adulthood, or from cultures with a sex-agnostic approach to raising children, often have an easier time with this stage, but it is not impossible even for those who have reached maturity as their observed sex.

Full integration

The final stage fixes the transformation. After this, the ritual is irreversible. Glyphs are marked on the body via tattoos, inlay, or scarification, replacing the need for potencies. The ðidngi and their elder travel into the wildnerness for several weeks, until the reach a location that appeared to the ðidngi in a dream. This is often a cave, hollow tree, or other cthonic setting. The ðidngi takes a final, ritualistically prepared potency and climbs into the cave alone, entering the Dream. The elder remains outside, keeping a fire lit to guide the ðidngi home. In the Dream, the ðidngi experiences an unmaking and recreation. The actual events are intense and unique to each individual, and the only consistent component is that they must sacrifice their old identity to create the raw matter to forge a new one. Upon emerging from the cave, the ðidngi has transformed, their body congruent with their known sex. Following the ritual, they assume their known sex for all arcane, legal, and social matters.