In 2269, after the ship's visit to Phi-11, Collins was part of a security detail which took James T. Kirk to sickbay. With Collins and most of the crew possessed by the Ur, Christine Chapel distracted Collins long enough for Kirk to strangle him unconscious. (TOS - Year Four comic: "Issue 3")
In 2270, Collins was part of a landing party that explored the surface of an unexplored planet and discovered ruins of a decadent city full of pleasure palaces, along with James T. Kirk, Spock, Leonard McCoy, Nyota Uhura and Laura Breen. Exposure to alien machinery swapped the sexes of all but Spock. Collins had become a tiny blonde woman, and he regained consciousness while being carried by a handling robot.
After the ship escaped to Starbase 11, Kirk considered returning to the planet given the possibility that the machinery would re-swap their sexes. Collins considered remaining a woman but was willing to return, and was turned back into a man. (TOS - The New Voyages 2 short story: "The Procrustean Petard")