Mook Carryover

Big bads die. That is a fact of a story......most of the time. That being said, when the Big Bad dies too early in the story's narrative, when due to the Sorting Algorithm of Evil, a Klingon Promotion, or being on the wrong end of Eviler Than Thou, it quite expected that a new guy will somehow take his place, usually with his own Mooks to do his dirty work.

Except sometimes not.

Mook Carryover is when Mooks of the previous Big Bad (The Dragon, The Evil Genius, The Dark Chick, or just the Goldfish Poop Gang) end up working for the new guy. How they managed to avoid death by either the hand of The Hero or their new boss varies: they may have betrayed their own boss, or were only mostly killed by the hero, and pitched their resume to the new guy. Or they may have been always working for the new blood, and just pretending to be loyal to the other one. Whatever the case, they are a familiar face among a bunch of new villains. This trope can be considered the Inversion of Replacement Mooks.

Whether they are still effective or not depends on which they remembered to keep their skills sharp, assuming they were very skilled in the first place.

Comics

 * The mooks of the Thunderbolts are just guys in cool armor with guns. When Swordsman bribes/hires a bunch of them for himself, while staging a coup against Norman Osborn, he spraypaints them in his colors. They don't last long, but they are mooks who get a palette swap and a new boss.
 * Star Wars: Legacy, when Darth Krayt dies, the entire Sith Empire works for Darth Wyyrlok for a while.
 * Manute of Sin City goes to work for Wallenquist after Ava Lord's death.

Film

 * Jaws somehow survives being dropped into a shark tank in the middle of the ocean at the end of The Spy Who Loved Me. When the original Dragon is killed in Moonraker, Jaws is hired as his replacement.
 * The character Ogre in Revenge of the Nerds shows up in the sequel in a different chapter of the Alpha Beta fraternity. The brothers he had in the original film are all gone, apparently defeated. Although later,
 * Lobo, the hulking henchman played by Tor Johnson in Bride of the Monster shows up again under a different Big Bad in Night Of The Ghouls, heavily scarred and apparently undead.
 * Throughout the course of The Dark Knight Saga, many of the mobsters goons turn to The Joker (after some "auditions").
 * Star Wars has an interesting one. The clones were created by the Jedi, and they continued to serve the Republic Empire, and one Jedi in particular. Also, pretty much all the Imperials in the Expanded Universe who don't fall under Defector From Decadence.
 * In X Men First Class, Magneto inherits Sebastian Shaw's henchmen after his Face Heel Turn.

Literature

 * In the Sword of Truth series, after Darken Rahl, ruler of D'Hara, is killed in the first book, large portions of his armies end up working as the expeditionary forces for the Big Bad for the rest of the series.
 * Also semi-inverted, in that the rest of Rahl's armies pledge allegience to the hero, Richard, after Rahl is killed. Richard at first believes this is a simple case of You Kill It, You Bought It, but later finds out
 * In Mistborn, after  death, many of his most powerful minions, notably Koloss and Inquisitors, start working for new Big Bad  . Justified in this case because the Lord Ruler created these beings and built a psychic "back door" into them so that he could always control them; with him out of the way, the far more powerful   was equally capable of exploiting said "door". His human minions are a much more mixed bag, with some joining the heroes and others various Big Bad Wannabe kings.
 * Across JRR Tolkien's Middle-Earth, many of the same creatures that once served Morgoth transferred their loyalty to Sauron when he took over as Big Bad. In this case, that would be because Sauron was The Dragon to Morgoth, so most of these minions would have been used to taking orders from him anyway.

Live Action TV

 * Mighty Morphin Power Rangers had Lord Zedd usurp Rita Repulsa as the series villain, but he kept all of Rita's old henchmen under his employ. He even upgraded the Putties to be tougher, provided nobody hit the obvious emblem target on their chests.
 * Mahou Sentai Magiranger has Vancuria, who was immortal and couldn't be killed anyway. So when a new villain took control of the Infershia army, she always stuck around. In fact, because of said immortality, and a Heel Face Turn, she outlasts almost all of them. The other Dragon, Wolzard, was similar, though he did go rogue a few times, and the various Infershia leaders used the same army of Mooks. Power Rangers Mystic Force did the same thing.
 * The Bibi soldiers from Tensou Sentai Goseiger.
 * In Kaizoku Sentai Gokaiger the Movie The Flying Ghost Ship, nearly all the mooks from all over the Super Sentai franchise appear as a single army to fight the Gokaigers.
 * At the end of Buffy the Vampire Slayer Season 1, she kills The Master, but the Annointed One is still around to be the Big Bad of Season 2 -- except early in Season 2, Spike comes along and kills him, taking over his operation.
 * Officer Braca (later Lieutenant) in Farscape was second-in-command to most of the series' big bads, in order. He just kept trading up.
 * Twenty Four after Erwich is killed by his superior, his remaining goons turn to him.
 * In Stargate SG 1, it's normal for a Goa'uld to take over a fallen Goa'uld's troops (even if he killed the former owner of the army. The soldiers see the Goa'uld as gods, and so wouldn't rebel against any of them.) Apophis shows up with Sokar's "Red Guard" as well as his usual Serpent Guard after he makes the rival who'd captured him pay for not just shooting him, and Hathor's second appearance has some of Apophis' Serpent Guards along with the Horus Guards you'd expect her to have. (She's known to use Brainwashing, though.)

Video Games

 * Professor Layton and The Unwound Future has the "worked for the true villain" kind.
 * Revolver Ocelot. Throughout the first three games (Metal Gear Solid, Metal Gear Solid 2 Sons of Liberty, and Metal Gear Solid 3 Snake Eater), he is working for the Big Bad as The Dragon. And at the end, he reveals that he was really working for someone else (and the Third game is a Prequel, so he's be at this for a while. This changes in Metal Gear Solid 4 Guns of the Patriots, where he finally decides it's time to be the big bad himself. Except he's not. It's complicated because it's Ocelot.
 * Epic Battle Fantasy has mooks from the first two games appear in the third game as Akron's mooks.
 * In Overlord you're invoking this trope by taking control of the previous Overlord's tower and minions.  In fact there have been a whole chain of Overlords over the years, but they all use the same sort of Minions and even a couple of the same individuals
 * Final Fight and its SNES sequels has passed the Big Bad baton from Belger's Mad Gear Gang (the first game) to an international subsidiary of Mad Gear run by Belger's (previously unknown) Dragon Retu (2) to Black and his Skull Cross Gang (3). Regardless of the main villain, the Andore family is always under the employment of these gangs, and as such, are the only recurring enemies throughout the series.
 * In Digital Devil Saga, Bat is originally a mook working for a Tribe you are fighting to defeat in order to form an alliance with them, but then later joins another Tribe and then kinda sort of does his own thing (becoming a Big Bad in his own right).
 * In Mario and Luigi Bowsers Inside Story, Bowser encounters a few Monty moles who used to work for him. Turns out they've gone over to Fawful's side.

Western Animation

 * Jackie Chan Adventures had the Enforcers; Finn, Ratso, and Chow, who were always brought in by the current seasons big bad. Valmont, Shendu (possessing Valmont), Daolon Wong, Tarakudo, and finally Shendu's son Drago. The latter averts it however, by firing them not long after.
 * Inverted with the Shadowkhan, who are introduced as the Elite Mooks of Shendu, but are revealed to have orignally served Tarakudo; Shendu basically stole them.
 * A flashback in an episode of The Venture Brothers reveals that Henchman 24 previously worked for Phantom Limb (the then henchmen 9, the future Monarch, promises to make him his Henchman #1 someday). As they all are part of the larger Guild of Calamitous Intent, it's implied that a henchman might switch villains during the course of their henching career.
 * In WITCH, at the end of season 1, a large number of Phobos' orcs defect to rebels, as Phobos didn't treat them terribly well, either. Come season 2, and the orcs now make up a major part of Elyon's army.
 * For a more villainous example, a number of Phobos' named minions decide to form a group called the Knights of Vengeance under the guidance of new Big Bad Nerissa. She continues to use them until she's powerful enough to create a Quirky Miniboss Squad that's entirely her own.