While I’ve never adapted fan fictions into originals (I don’t write fan fiction), I do know parts of how it works. But it all depends on the fan fiction you write.
After, for example, was pretty easy to adapt as it was just changing names. It was a One Direction fan fiction where the love interest was Harry Styles. If Anna changed Harry’s name, it’d instantly be an original fiction. And that’s exactly what happened when the book became traditionally published. Harry Styles became Hardin Scott. And that was it.
But it’s different with a fan fiction from a book, movie, or TV show because then you’re dealing with characters, settings, and more that isn’t yours. However, there is a way to figure out what to do based on what elements you used in the fan fiction.
If you used characters that weren’t your own, then you can rename and recreate those characters (their personalities, etc.) to make them a little more original. For example, let’s say you wrote a Harry Potter fan fiction where your MC, Noah, goes to Hogwarts with Harry and the gang. To make it original, you’d have to rename the characters and make their personalities a little different. And, if you’d like to go deeper, then you can also change their backstories as well. For instance, Harry could turn into a Max; Hermonie into Emma, and Ron into Lewis. For personality changes, you could make Max into a selfish guy where he feels like he’s all that. He may have grown up with his dad (and his mom died). Emma could still be the intelligent character out of the group, but she’s extremely shy and not like the Hermonie we know and love. Lewis, on the other hand, could be an only child and is very outgoing.
Changing their characters is a part of adding that originality because if you keep them very similar to the original story, then it’ll look like you copied off it. Which is why I gave the example where Max (AKA Harry) is wanting the spotlight whereas the original Harry didn’t really care for it.
If you used a setting that is from the original story, then you would need to scrap it and make it your own because then if it’s fully there, then you’re copying it basically. So, in regards to my Harry Potter example, instead of Noah and the gang going to Hogwarts—a school of witchcraft and wizardry with four houses (Slytherin, etc.) and acting as a private school—then you need to put your own spin on things.
For instance, let’s say Noah and the gang go to a school for learning magic (as it’s been done before, so it’s not much of a copying thing), but instead it’s just for witches and wizards, it can be for all creatures such as goblins, fairies, etc. that are intellectual. And instead of it acting as like a private boarding school, it could be a simple public school (without houses). You also would need to change the actual setting. Hogwarts, for example, is in Scotland and it’s next to a lake. If you wanted to make it original, you’d need to change it up a bit. You could put your school next to a lake, but you can also be more original. So you could put your school on an island where it doesn’t just have lakes, but is surrounded by the ocean.
To go deeper, you’d also need to make sure the architecture is different. So, for example, instead of having a brick castle, it could be a large drywall building. Or maybe it’s glass walls. And instead of moving staircases (which is one of the popular ideas from the book), it could be non-moving or there are no staircases and everything becomes a portal to the rooms or something of that sort.
You also would need to make sure the slang and other kinds of similarities are different or gone. For example, if you have a Harry Potter fan fiction that turns into an original story, you can’t have characters calling non-magical people “muggles,” or if a character comes from one magical parent and one that isn’t, they can’t be “mudbloods.” You have to create your own slang or drop it from your story.
Another thing you need to do is to make a different plotline than the original book. Of course, this may be already done in your fan fiction, but it’s something to note if your story takes place with a similar timeline. So, for instance, Noah can’t be tracking Max to see the status of how he’s doing with the Dark Lord. The story would have to be something where Noah’s journey is the main focus. What does he do in this world? Could what he do have a different effect in the end with the Dark Lord? You have to think about what happens in the story with all the characters and how similar they are from the original story you took it from. This way, there is little to no similarities with either story.
And finally, understand that a story becomes more original by the plot twists and differences you use in your story. So, for instance, you could follow the guideline that J.K Rowling uses where the hero wins and everything is good. But you could be creative to a point where the hero may not win or may not win in the sense that everyone believes (such as killing off the bad guy and getting the girl). You can also add more turns in your story that puts it in a different place than the original. For example, in the first Harry Potter book, one of the main twists was how a teacher was Voldemort’s ally (and he used the teacher to keep him alive). Instead of using a similar tactic (where someone we less expected to be evil), you could make the evil person obvious (like how J.K used Lucius Malfoy as one of the allies to Voldemort) and work around that to make it more twisty (like perhaps they want to be good instead).
Overall, it’s just a matter of changing everything you used so it’s not obvious that it was a fan fiction. Some fan fictions are easier to hide those similarities such as After and 50 Shades of Grey. 50 Shades was a Twilight fan fiction, but much like After, all E.L James did was use the characters from Twilight: Bella and Edward. All she had to do was change the names. But for someone who wrote a fan fiction that was based in the setting or lifestyle of another book, movie, TV show, video game, etc., it’s a lot harder to hide because you have to look for all the elements you used and change them so it’s more of your own story. If that makes any sense?