What changes could you make to a specific movie that would change it into a horror movie?

The 5th Element, but she decided the world isn't worth to be rescued.
Fight Club but the roles are switched about who is imaginary.
Jurassic Park, but getting eaten as a guest is intended.

And romcom with a male lead finding love, but the manic pixie is fat and unattractive. She'll go from quirky to annoying stalker real quick.
- Baby Reindeer is kinda that.

I feel like having Wallace in A Grand Day Out always having similar style of movement as the scene where he's hammering something would make the already somewhat trippy, in my opinion, short film feel more like a horror movie. The way he just stands perfectly still and doesn't react while his arm flails back and forth with the hammer in his hand just feels menacing in a way that would make the rest of the movie more horrifying, in my opinion, if all his movements were like that.

Any romcom where the handsome guy is trying to get the girl: make the guy unattractive.
- So like any Adam Sandler movie?
- Or just add scary music when the guy is onscreen.

I remember watching a video where Corridor turned Willy Wonka and the Chocolate Factory into a horror movie by making a few cuts, changing the music and doing some visual effects!
Content warning! Goes without saying but there's some blood and gore in this one!

Nerdwriter once made a video about how Passengers could have been a horror/thriller (and a better movie) if it had been told through the point of view of Jennifer Lawrence instead of Chris Pratt's. https://www.youtube.com/watch?v=Gksxu-yeWcU
- One of the comments there says this would be similar to Moon and countless other movies. But a big Blockbuster Moon/TwilightZone-style flick sounds good to me. I've been dying for a Star Trek movie that's more weird Twilight Zone scifi plot and a lot less save the universe/Federation from being wiped out.
  
  You've got a few episodes of Love Death and Robots that really have that twilight Zone feel (and some are set in space!). That's also the case for the pacman episode of Secret Level. This one gave me chills down my spine. 😅

Take any action movie and really focus on those kills, especially when it's with melee weapons instead on guns. Don't even need to go full splatter movie, just actually show the violence that's happening in great detail and with maybe some amount of realism (people rarely die in one blow, and bladed weapons can get stuck in bodies)(obviously that doesn't work as well for sci-fi and superhero movies). There are going to be a lot of turned stomachs.
A lot of spy movies, just don't fade to black when there's torture.

Sonic the hedgehog by using the old sonic design
- I'd still like to believe they did that design as a marketing stunt to get people talking about it, but I also have a lot of doubt they would have been smart enough to actually come up with something like that..

Jack Ryan doesn't figure out Marko Ramius is wanting to defect. (Hunt for Red October)

Jack in the movie Titanic is actually a serial killer. Rose is his accomplice. Dead bodies keep showing up throughout the ship, people start to panic before the iceberg. The ship still hits the iceberg and still sinks. His final kill is kathy bates's character. She was the lady in the actual movie that was actually nice to him and helped him get a nice suit. (If my memory is correct). Don't even need to go full gore.

Someone recast Elf as a thriller: https://youtube.com/watch?v=EkwdYSn3Uws
Not quite what you're asking, but similar energy I think!