I continued a fanfic after I left it to stew for two years. I am not convinced that my old readers saw it, but that is fine. Still, I like the new ending way better than the old one. The important thing is to read what you already have beforehand. Even if you have taken notes, you can't remember every detail, and the notes won't be as descriptive as the story itself. Just don't rewrite, if you can't rewrite. You will change too much, and then it will be a different story, with a different soul.