How close to the original Peter Pan story does a rework need to be to count as fanfiction?

And when does it just become an original story inspired by the larger Peter Pan trope? Do the character names need to be the same, the origins? Does it have to take place in Neverland? What about if you add in other elements, like the immortal youth is because of vampires or something?

I would say Peter Pan is one of the most common out of copy-write stories for "official" fanfiction that gets adapted, and from what I've seen they range from pretty close to the original with just a few changes (like aging them up), or vastly reimagined (Brom's The Chlid Thief, which I do NOT recommend).