@Spi:
Let me see if I can explain this a little more clearly for you.
In your story, you used large chunks of EL James' "50 Shades of Grey" and changed the character names to the names of the characters from Harry Potter that you wanted to use. Just changing the names of the characters is not enough to make that your own writing. It's still EL James' words you're using, and each of those sentences of her that you use like that has to be cited.
The correction you made is NOT a citation. You just added another disclaimer at the end. You should be familiar with the format of citations from literature classes as far back as high school, and most certainly in college. A citation is how you acknowledge a direct quote from another author, and that's what you need to do. You need to mark EACH line or paragraph that is EL James' writing with something like a [1] or even an *, even if you've changed that character names, and at the very bottom of the chapter, you need a footnote that looks like this:
[1] EL James, "50 Shades of Grey", Chapter 1
That way, you are acknowledging that you did not write those words yourself, and you are giving the proper credit to EL James, who did write those words.
Just so we are very clear here, if you do NOT properly cite EL James' words that you have used, it is considered plagiarism, which is a crime. EL James, her publishers and their lawyers will take a very dim view of this, and could seek legal action against you, and against us if we allow this to remain in our archive. If you don't make the correction, the story needs to be removed from our site.
Please let me know if you need any further explanation of what needs to be done,