The Changed (also referred to as Changed Humans or Changeling Humans) were a type of humanoid beings, a variant of Humanity created through genetic engineering, designed to survive in hostile colony environments.
Jenniver Aristeides was a Changed who served on the USS Enterprise in security and later botany. Her species was a form of genetically engineered human created to be tall (250 centimeters tall) and immune to most everything. (TOS novel: The Entropy Effect)
One Changed humanoid was a Starfleet guard in 2271 at Starfleet Headquarters. (TOS - The Lost Years novel: The Lost Years)