The processes transgender people go through to match their gender identity, which may include sex reassignment or gender confirmation procedures, but not necessarily. (Acceptable uses: Washington is transitioning while helping his daughter consider universities. Chamberlain’s family offered support during her transition.)
Avoid the phrase sex change.
Altering one’s birth sex is not a one-step procedure; it is a complex process that occurs over a long period of time. Transition can include some or all of the following personal, medical, and legal steps: telling one’s family, friends, and co-workers; using a different name and new pronouns; dressing differently; changing one’s name and/or sex on legal documents; hormone therapy; and possibly (though not always) one or more types of surgery. The exact steps involved in transition vary from person to person.
(Sources: AP Stylebook, GLAAD Media Reference Guide)