How many per cent of the text needs to be changed?
Fanfictions are original works using the setting or characters of another work.
If you change some things and republish, that’s an adaptation.
For example, the recent film Fire Island was an adaptation of Jane Austen’s Pride and Prejudice.
There isn’t a legal right of fair use for fan fiction, if that’s what you’re asking. Rights holders often ignore it, though.
This is a variation of the famous Ship of Theseus thought experiment.
Well it’s not because there is a legal answer.
Not sure you understand what fanfiction even is.
Nice try, DC.
100%
Well, the answer varies a lot based on the copyright owner. If it’s something based on LOTR then the answer is a lot, but if it’s something in the public domain or with a copyright owner that’s not too strict then you may be able to keep a lot of similarities.