Dictionary.com
definitions

sex change

noun
  1. the alteration, by surgery and hormone treatments, of a person's physical sex characteristics to approximate those of the opposite sex: Born male, she now lives as a woman but has no plans for a sex change.

Origin of sex change

First recorded in 1975–80
Also called sex reassignment, gender reassignment.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for gender reassignment

gender reassignment

noun
  1. male-to-female or female-to-male transformation involving surgery and hormone treatment

sex change

noun
    1. a change in a person's physical sexual characteristics to those of the opposite sex, often achieved by surgery
    2. (as modifier)a sex-change operation
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
gender reassignment in Medicine

sex change

n.
  1. The modification of biological sex characteristics, by surgery and hormone treatment, to approximate those of the opposite sex.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.