You should probably check if there could be legal trouble .

I think you should. When you publish it it's not a fanfiction anymore.

you should probably change the idol's name to avoid getting sued. but i think you should keep jungkook as jungkook and just change his last name. jungkook jung, jungkook kim, etc because idk, i can't picture jungkook as not being jungkook. but it's up to you.

It would be identity theft if you didn't change the names btw

I'm my personal opinion I would rather you used K-Pop names but in my honest o permission from opinion, wouldn't you get sued for using their names without permission from BigHit? It's the same as using there songs on youtube or in a game. With out paying you would get in trouble because we are buying it not getting free books,

well in my opinion, you could but i just think people would prefer the kpop idol's name, i mean that's why we all read the story to begin with :)
i wouldn't lose interest in the story because of a name change, but i know others who will.