Card evolutions

Retrained is the term used to describe a monster that has been reprinted with a new effect and slightly different name, but with the same base ATK and DEF, and usually Level as well. Examples include "Battle Ox" to "Enraged Battle Ox" and "Magician of Black Chaos" to "Dark Magician of Chaos". Special cases are Harpie Lady 1, Harpie Lady 2, and Harpie Lady 3, which all have the same stats as the original Harpie Lady, but are not all representing the same creature. Rather, they are the three sisters from Harpie Lady Sisters.

The term "retrained" comes from "Retrained Elf Swordsman," a translation of the Japanese name for "Obnoxious Celtic Guard", who is a retrained version of "Elf Swordsman" ("Celtic Guardian").

Retrained monsters are not to be confused with upgraded ones, such as Cannon Soldier MK-2. These, like retrained monsters, are clearly based on existing cards. However, unlike retrained versions, these have higher ATK and/or DEF and an effect meant to improve upon the original.

Retrained Cards

 * Harpie Lady 1 -> Harpie Lady 2 -> Harpie Lady 3 -> Harpie Lady -> Harpie Lady Sisters
 * Battle Ox -> Enraged Battle Ox
 * Magician of Black Chaos -> Dark Magician of Chaos
 * Black Luster Soldier -> Black Luster Soldier - Envoy of the Beginning
 * Celtic Guardian -> Obnoxious Celtic Guard
 * Doll of Demise -> Malice Doll of Demise
 * Gaia the Fierce Knight -> Swift Gaia the Fierce Knight
 * St. Joan -> Guardian Angel Joan
 * Summoned Skull -> Skull Archfiend of Lightning

Upgraded Cards

 * Cannon Soldier -> Cannon Soldier MK-2
 * Hane-Hane -> Hade-Hane
 * Machine King Prototype -> Machine King -> Perfect Machine King
 * Man-Eater Bug -> Nobleman-Eater Bug
 * Muka Muka -> Enraged Muka Muka

Transformed Cards

 * Aqua Madoor -> Neo Aqua Madoor
 * Blue-Eyes White Dragon -> Blue-Eyes Toon Dragon
 * Dark Magician -> Dark Sage
 * Dark Magician -> Dark Eradicator Warlock
 * Dark Magician Girl -> Toon Dark Magician Girl
 * Gigobyte -> Gagagigo -> Giga Gagagigo -> Gogiga Gagagigo
 * Harpie's Pet Baby Dragon -> Harpie's Pet Dragon
 * Inpachi -> Blazing Inpachi -> Charcoal Inpachi -> Woodborg Inpachi
 * Levia-Dragon - Daedalus -> Ocean Dragon Lord - Neo-Daedalus
 * Petit Moth -> Larvae Moth Great Moth -> Perfectly Ultimate Great Moth
 * Red-Eyes B. Chick -> Red-Eyes B. Dragon -> Red-Eyes Darkness Dragon -> Red-Eyes Darkness Metal Dragon
 * Ryu-Ran -> Manga Ryu-Ran
 * Summoned Skull -> Toon Summoned Skull
 * The Creator Incarnate -> The Creator -> The Dark Creator
 * Vampire Lord -> Vampire Genesis
 * Warrior of Zera -> Archlord Zerato -> Darklord Zerato
 * Warrior of Zera -> Mazera DeVille
 * Warrior of Zera -> Zera the Mant
 * Helios - The Primordial Sun -> Helios Duo Megistus -> Helios Tris Megistus
 * Feral Imp -> Des Feral Imp

LV

 * Allure Queen LV3 -> Allure Queen LV5 -> Allure Queen LV7
 * Armed Dragon LV3 -> Armed Dragon LV5 -> Armed Dragon LV7 -> Armed Dragon LV10
 * Dark Lucius LV4 -> Dark Lucius LV6 -> Dark Lucius LV8
 * Dark Mimic LV1 -> Dark Mimic LV3
 * Horus the Black Flame Dragon LV4 -> Horus the Black Flame Dragon LV6 -> Horus the Black Flame Dragon LV8
 * Mystic Swordsman LV2 -> Mystic Swordsman LV4 -> Mystic Swordsman LV6
 * Silent Magician LV4 -> Silent Magician LV8
 * Silent Swordsman LV3 -> Silent Swordsman LV5 -> Silent Swordsman LV7
 * Ultimate Insect LV1 -> Ultimate Insect LV3 -> Ultimate Insect LV5 -> Ultimate Insect LV7
 * Winged Kuriboh -> Winged Kuriboh LV9 -> Winged Kuriboh LV10

Metalmorph

 * Red-Eyes B. Chick -> Red-Eyes B. Dragon -> Red-Eyes Black Metal Dragon
 * Zoa -> Metalzoa
 * Musician King -> Heavy Metal King