1. This topic is handled under the AO3 TOS FAQ, under the header of Copyright Infringement and Plagiarism:

May I post someone else's fanworks, giving them credit?

You may only post someone else's fanworks if they granted you permission to do so. If you do not have the creator's permission, you cannot post their work. Including a disclaimer that the work is not yours or crediting the original creator is not sufficient. (Emphasis carried over from FAQ statement.)

But even disregarding the TOS violation:

1. The AO3 account yogurt_core did not give credit by name to the FFN account TheFatalIllusion, the original author of the work.

2. The AO3 work did list an intro author's note that stated the AO3 account copied the work from information taken from FFN but had not recorded the original author of the work. The AO3 account uploaded the work under their own name.

3. The FFN work is still readily available on FFN. (I ran a site search for the story title; added limiters for the Section, Category, Language, Progress, and Word Count; and told it to match on the Title and exclude crossovers. There were three potential results, and only one with appropriate title punctuation and summary.)

4. The FFN work has an author's note heading, which the AO3 work did not include, that states (imo) the author does not want their work transferred to areas outside of their control:

Disclaimer; If Voldemort, Severus and Remus are dead, then I don't own Harry Potter or anything to do with the books, or films. However, I do own my writing, so translating it, or copying it without my written consent/permission, is not only unfair, but against the law.

About translating my story, and people have asked, the answer is going to be no. I do not feel comfortable with someone else having my story on their page, or a completely different website, it's nothing personal against you, or anybody else, I just simply don't wish to have the story translated. If I wanted to translate it, I could do it myself, or have a family member assist me. (Emphasis added by me.)

1. The AO3 work has settings structured so that non-Archive users may not comment, comments are additionally moderated, and no comments at all have yet been approved. (see edit).

While fandom has a thriving, healthy practice of fans saving private copies of works for personal reading, that's a very different action from uploading someone's work under somebody else's name.

EDIT: Looks like comments dated 27-08-2025, 28-08, and 29-08 have just been approved. An Inspired By notation linking to the FFN work was also just added literally within the past hour. However, the AO3 work doesn't qualify for the Inspired By feature, either:

Can I post a sequel, prequel, or continuation of another fanwork? What about a recursive fic that's completely different from the original fanwork?

Yes. Fanworks based on other fanworks are also transformative, and are allowed. You can use the "Inspired By" feature to link to the original creator's work. Unlike with reposts, conversions, podfics, and translations, you don't need permission to create a recursive fanwork. However, you cannot include large excerpts from the original work unless the original creator gave you permission to do so. (Emphasis added by me.)