In "Girl Powdering Her Neck," what is the young woman in the poem transforming herself into?
Literally speaking, the young woman in the poem is transforming herself into a courtesan, a woman who is, to some degree, selling her body. In the Japan of these times, there were women who were not exactly prostitutes, but whose function was to bring pleasure (at times physical, at times aesthetic) to men. This is what the woman in the poem is doing. We can tell this from the following stanza (and because we know the subject of the artwork the poem is based on)
Morning begins the ritual
wheel of the body,
the application of translucent skins.
She practices pleasure:
the pressure of three fingertips
Fingerprints of pollen
some other hand will trace.
She is making herself beautiful the way she does as a daily ritual. The word "pleasure" appears, and we are told that it is for the benefit of others that the transformation is occuring.
Looked at slightly differently, the woman is transforming herself from a person (the individual she is on her own time) to an idea. She is becoming an ideal woman as defined by the men of her society.