Split Personality Makeover

When dealing with a character that has a Split Personality, there will frequently be a striking visual difference depending on who is "in control." It's not a literal transformation, like the original Jekyll and Hyde; it's still recognizably the same person, but the differences will be visible. This may include anything from subtle stylistic changes (the character's design becomes more hard and angular when an evil or aggressive personality is in control, but softer and more rounded when a kind and good personality is in control) to more physical things like changes in hairstyle and black bags under the eyes. A significant vocal shift is common as well.

Some of these changes are perfectly possible to do without physically altering the person's body, like the personalities having hair that's styled differently (or one of them not styling it at all and another styling it neatly) but the same length, or changes in body language and posture, or talking differently without completely changing voice actors, but others are firmly in the realm of Artistic License.

These changes may or may not be noticed in-universe; if they are, expect them to treat the change as something intangible. They'll comment "he's like someone completely different now!" or "that's not the person I know!" rather than "hey, when did you get your hair cut?" or "did your voice just drop two octaves?".

Anime and Manga

 * Yugi from Yu-Gi-Oh! is one of the most prominent examples; when his "playing card games" personality takes over, his hairstyle changes, his eyes switch from Tareme Eyes to Tsurime Eyes, his voice deepens significantly, and he's drawn to appear taller and more imposing. (The height change only happens in the anime, though, and in Japanese he kept the sae voice actor, who just changed her delivery style.) Happens with Bakura and Marik too. Yugi and Bakura are somewhat Justified due to literal magic being involved, but Marik just has a mundane Split Personality, giving him no excuse.
 * The personality isn't magically implanted, but he has considerable magic at his disposal, and split personalities have been associated with magic since forever. So whatever.
 * Allelujah Haptism from Mobile Suit Gundam 00 is a more subtle example; which personality is in control depends on which of his Mismatched Eyes is hidden by his Peek-a-Bangs. Unusually for this trope, he's actually seen shifting his hair around in-universe when he changes personalities, rather than it happening automatically as a stylistic change.
 * Jamie from Zoids New Century gains swept back hair and a bold, deep voice when he becomes the Wild Eagle during air combat.
 * You can tell whether Agito or Akito is in control on Air Gear depending on which eye their eye patch is on. They also have differently shaped pupils, and in the anime, drastically different voices..
 * In Psyren, Abyss has darker skin than.
 * Motoko, the female protagonist of Change 123. (Just look at the picture on the linked page!)
 * Lucy, the main character in Elfen Lied has rounder eyes, child-like and a softer expression as her second personality Nyu.
 * Razlo and Livio from Trigun Maximum have a system with fairly reasonable in-universe mechanics. When Livio's running things, he has his long hair flipped over on side of his face. When Razlo takes over, he flips it to the other side and we discover that the previously covered side of his head is bald with weird computer-chip tattoos and a wild eye.
 * The manga 'Historie,' a biography of one of Alexander the Great's secretaries, starts weird, and when Alexander finally appears gets weirder. Apparently Alexander is a rather sweet little prince who developed a split personality upon walking in on his mother having an affair with a slave. That split personality is (Alexander's real-life best friend) Hephaestian, who is kind of a dick and whose presence is signified by a fixed stare and covering up Alexander's snaked-shaped forehead birthmark with his mother's makeup.
 * Lady Une in Gundam Wing has a split personality (which Treize claims is his fault), and the Colonel and the Diplomat can be distinguished easily by the former having her hair up and wearing glasses, and the latter wearing it down and taking the glasses off, as well as changes in facial expression and posture.
 * Sensui, the biggest single villain in Yu Yu Hakusho (and the mangaka's favorite) has seven personalities. We meet three, and the visible difference is as impressive as it is subtle, having as much to do with mannerisms and speech patterns as it does visual cues.

Film

 * Buddy Love in the original version of The Nutty Professor.
 * Gollum from Lord of the Rings qualifies - not only does his voice change, but his appearance is also subtly different.
 * Willem Defoe does a masterful job of this playing Norman Osborne/Green Goblin in the first Spider-Man movie. Using only facial expression, vocal changes and body language, he becomes a completely different character, Nightmare Fuel incarnate.
 * Not a split personality, but when Mulan is dressed as a soldier, she is drawn with no eyelid folds and a more angular jaw to make her look more masculine.

Literature

 * Thendy Bravura (also known as another dozen names) in Felsic Current incurs very visible personality changes. Often, others can tell he's just "done it again" merely by witnessing his facials features melt into a completely different countenance, before he even speaks. Their mounting exasperation to these frequent incidences is usually the source of comedy.
 * The dark Cinderella adaptation Sunny Ella plays with this, as Ella sees the world (and herself) one way but the reality of the situation is much different. Ella sees herself as a beautiful fairy tale heroine with a lovely singing voice and a sunny disposition; in reality she is mute, scarred, unkempt and has violent tendencies.

Live Action Television

 * They comment on this extensively in the miniseries Jekyll. Hyde is slightly taller and thinner than Jackman, and he also has a slightly different hairline. Of course it's the same actor plus platform shoes and different tailoring/shooting angles, but the effect is subtle and present.

Video Games

 * Therese Voerman from Vampire: The Masquerade Bloodlines is a stiff Ventrue Ice Queen in a business suit. Her "twin sister" Jeanette is a slutted-up Malkavian party girl. You find our they're sharing the same body when you reach the end of their storyline, and find Therese/Jeanette wearing Therese's clothes but with her makeup and hairstyle split between Therese's professional do and Jeanette's party girl do.
 * .hack//G.U.'s lets you know who is in control by  Given that this is an MMORPG, this is hidden under the guise of