You've got a story about a teenaged orphan who's living with his aunt and uncle. He does on a quest where he meets a sage mentor who knew his father and said his father was killed by the story's antagonist who in turn murders his aunt and uncle. He goes on to rescue the damsel in distress and achieve his quest. Cool.
Two sequels go on to show the quest encounter various reversals before it unfolds into a larger crusade to defeat the focus of evil in the world. One major complication in the story is that the boy's father turns out to be the antagonist who'd been turned evil. Yes, Darth Vader is Luke's father who's gone bad and ultimately repents of evil and receives a modicum of redemption.
It's not a bad story arc over the course of the three installments.
But, you may wonder, how did Darth Vader become evil in the first place? The answer is held in not one, but three prequels. But while consuming millions of dollars worth of CGI animations and all manner of explosions, sword fights, and derring-do, we all know that Anakin is going to break bad.
Seems sort of pointless, doesn't it? You like the girl? She's gonna die. You like the annoying kid, and the petulant teen? He's gonna be wearing the black helmet. Why bother investing in the story along the way when you know where it's going?
Play him off Keyboard Cat.
When I watched the second Indiana Jones movie, I thought it was a SEQUEL to the first. Thus I was on board with the fight scene where the two guys with swords go after him, and he reaches for his gun and it's not there. It was cute, because if this fight scene happens after the first fight scene with one guy with a sword that Indiana Jones dispatches with a gunshot, then his reaction and my reaction makes sense.
However, I recently learned that Temple of Doom was a PREQUEL to Raiders. How is it that Indiana Jones could react as he did when the two sword fighters dressed identically to the one sword fighter confront him? Had I known this was a prequel, it would have utterly taken me out of the story. In fact, this is such a violation of continuity, that I'll never be able to watch these stories again. The whole franchise is dead to me. There's a certain amount of suspension of disbelief that's good and reasonable, but there are limits.
So, you've finished a well-received work and you're considering another writing project that's set in the same world. You have thought through the back-story of all your characters and you're proud of how well all that scaffolding holds together. You may have even written a few scenes with the detective's dead partner that were key in developing his character.
You don't publish scaffolding.
Instead of turning that scaffolding into a prequel, turn it into a sequel. Maybe Spade and Archer did something special before they had a case involving a black bird from Malta. Then carry it forward a generation after Sam sends Bridgid up the river. Then put together some young punk with those old clues from a generation before and solve the case in a context where the reader doesn't know for sure who's coming out of the story alive and who isn't.
You know that Saul Goodman is an amoral lawyer with a dark sense of humor when you meet him in the second season of Breaking Bad. And you know he has a rolodex of guys who know guys who an make any unlawful thing happen. If you start telling his story when he's in law school, you know he's going to end up passing the bar. If he starts with a girlfriend who he thinks is "the one" you know they'll part company. If he has any shred of morality and idealism, you know it'll be gone by the time he meets Walter White. These things are foregone conclusions.
Instead, I'd like to know what becomes of him after he's managing a Cinnabon in Omaha. Or when one of Jesse's burnout buddies gets a job in that Cinnabon. What's past is past. Tell me what happens next.