Little is known of the life of mutant shape-changer Kevin Sidney before he became second-in-command of the subversive organization Factor Three and adopted the alias Changeling. After discovering the true plot by Factor Three’s leader, the Mutant Master, to wipe out both man- and mutantkind, Changeling freed Professor Charles Xavier, the telepathic founder of the team of mutant heroes the X-Men, and the sonic-screaming mutant known as Banshee who had been captured by the organization. Banshee unmasked the Mutant Master as a member of the alien Siri race, and the X-Men and Factor Three’s mutant agents joined forces to defeat him.
Shortly thereafter, Changeling discovered that he had terminal cancer. Seeking to make amends for his past, Changeling approached Xavier, who asked Changeling to impersonate him while he went into seclusion to prepare a means of repelling an approaching invasion by the alien Z’nox.
While using his shape-shifting powers to pose as Xavier, Changeling heroically died while thwarting an attempt by the subterranean menace Grotesk, lone survivor of a subterranean race who sought to destroy the Earth, when an oscillotron machine exploded, mortally wounding him. Changeling was impersonating Xavier at the time, and as such the X-Men believed their mentor dead until the real Professor X eventually returned.
Later, the voodoo practitioner known as the Black Talon resurrected Changeling as a zombie and forced him to serve in a team of other mutant zombies called the X-Humed. Using his powers to impersonate the great musician Elvis Presley, Changeling was able to draw attention to the X-Humed’s activities. After the gamma-spawned adventurer She-Hulk defeated the Black Talon, Changeling once again became truly dead.
Changeling then seemingly returned as a malevolent ghost to haunt the British super-team Excalibur, however this was later revealed to be a hoax perpetrated by the sorcerer Merlyn.