The Borg -- arguably Star Trek's greatest villain -- rob humans and other races of personal identity, assimilating individuality into a colorless collective.
Similarly, vampires in Buffy the Vampire Slayer are demons who inhabit your (dead) body, but lack your human soul.
From one end of the genre to the other, villains in the pantheon might accurately be described as body thieves or identity robbers/identity corrupters.
In body swap tales, two individuals change bodies, and in the process overturn the order of the status quo. In sci-fi TV history, villains have swapped bodies with heroes, men have swapped bodies with women, and sane men have swapped bodies with mad-men.
And in each example of this template, the great struggle in the drama is to re-assert personal identity and reclaim a life that might have been lost. We cling to our identities. Without it, we are nothing.
The Joe Stefano, Leslie Stevens anthology The Outer Limits (1963-1965) featured an early "body swap" story during its first remarkable season.
In this case, the Spock-Kirk friendship proves paramount, and Spock makes use of a Vulcan mind-meld to prove that Captain Kirk's true self is trapped in the body of Dr. Lester. Again, the idea of the soul is raised, if not named directly. In "Turnabout Intruder," Captain Kirk speaks of the things that make him "special," "only to himself."
Chris Carter's The X-Files (1993-2002) took the idea of the "body swap" in a different direction entirely during the sixth season two-part episode, "Dreamland."
Here, Mulder is cut off from not just his body, his job, and his best friend, Scully, but from his obsessive, lifelong pursuits. He finds himself with teenage children, a nagging wife, and no real friends.
Morris, meanwhile -- at least in a certain sense -- does a better job with Mulder's life than Mulder did. As Mulder, Morris attempts to get frisky with Scully, and enjoys his new, more youthful and athletic body. Mulder gets moved into a suburban Hell, but Morris makes his time in Mulder's body enjoyable.
The Farscape canon (1999-2003) also features a variation on the familiar body swap story. In "Out of Their Minds" by Ian Watson, a Halosian energy blast strikes Moya and all the refugees and fugitives aboad her (save for Zhaan) are shunted out of their bodies...repeatedly.
At this point, Lionel is the undisputed villain of the series, and he returns to Clark's life in Smallville with super powers to go along with his criminal mind. The kicker in this case is that after order is restored and Lionel is returned to his own body, he feels the after-effects of Clark's presence. In an instant, Lionel is "born again," a reformed man. The switch has changed him, but not because of himself, but because his body housed a being of rare, superhuman virtue.
"Transference" points to another aspect of "body swap" stories that proves irresistible. Actors featured in a regular series are suddenly gifted with an opportunity to play a different role; to emulate their co-stars, in many cases.
Smallville proves truly spectacular in this regard, with Welling deftly taking on the gestures, stance and mannerisms of Glover's character, and Glover doing the same for Welling's character. That's the thing that makes this episode so funny, and it's really a credit to Welling (who gets most of the screen-time) for pulling off a brilliant, funny, and carefully-observed version of Lionel Luthor.
It would be terrible, after all, to lose your body to another person. Especially a person who might be you...better than you ever were.