The Iytean Change, often referred to as simply the Change, was a temporary physical transformation which Iytean symbionts could force upon their hosts, reshaping their very anatomy to better suit the symbiont's purpose. The Change would be reversed as soon as the Iytean relinquished mental control of the host, no matter how extensive the physical transformations. The Iyteans used it to give their animal hosts prehensile hands able to manipulate advanced technology. The Change was normally impossible to perform on sapient beings due to the difficulty of fully controlling another intelligent being's will completely. (GAME: "The Iytean Race" [+]Part of The Iytean Menace, J. Andrew Keith, The Doctor Who Role Playing Game (FASA, 1985).)
However, one renegade Iytean scientist discovered how to possess sapients using a "will-sapping drug", and perfected means of forcing the Change upon these hosts, turning their bodies to stronger, more brutish forms. (GAME: "Solving the Mystery" [+]Part of The Iytean Menace, J. Andrew Keith, The Doctor Who Role Playing Game (FASA, 1985).) After he found his way to 19th century London, this renegade's possession of Doctor Henry Jellicoe, and temporary Changes into a brutish man who adopted the alias of Ned Hines, served as the inspiration for Henry Jekyll and Edward Hyde in Robert Louis Stevenson's The Strange Case of Dr Jekyll and Mr Hyde. (GAME: "The Hunters Home From the Hill" [+]Part of The Iytean Menace, J. Andrew Keith, The Doctor Who Role Playing Game (FASA, 1985).) The various werewolves and lycanthropes of Earth folklore may also have been inspired by prior Iytean visitations to Earth during which other Iytean criminals forced Changes into monstrous forms on groups of possessed humans. (GAME: "Other Adventures" [+]Part of The Iytean Menace, J. Andrew Keith, The Doctor Who Role Playing Game (FASA, 1985).)