Don't know if OP was talking about this one but another BL manhwa have a REALLY similar background story: "I Became the Lousy Side Top".
- Transmigration into a BL story.
- Becoming the trash rich side characters that tormented the main character (Bottom).
- Turning the story around by becoming close the main character (and becoming the bottom in the process)
- Attracting the attention of the other main characters (Top) to himself.
- This part is quite hazy so take it with a grain of salt but I think remembering that this "lousy" side character also had a messy family.
So yeah, at first, I was thinking this story was about the first one - like I was re reading the same story - but the detail were off so it's then that I discovered they were two different stories... Was it plagiarism ? I don't know as I don't know the timeline of both manhwa or if they were inspired by a novel either. That being said, the detail of each stories are completely different and I do prefer this version of the plot.
What they said. https://www.mangago.zone/read-manga/i_became_the_lousy_side_top_official/ here you go

And suddenly became the bottom to the og bottom
I'd have two nickels. Which isn't a lot but it's weird that it happened twice