What is the name for sex change?

CHANGE in GENDER

a. a change in a person's physical sexual characteristics to those of the opposite sex, often achieved by surgery

b. ( as modifier ): a sex-change operation

c. The modification of biological sex characteristics, by surgery and hormone treatment, to approximate those of the opposite sex.

The surgical term for this is Gender Re-assignment Surgery, or Gender Re-alignment. (GRS)

The whole process, from changing ones name, through to living in ones preferred gender, to receiving Hormone Replacement Therapy (HRT), hair removal (in Male to female-MTF), voice training etc and finally GRS is known as Transitioning.