No, I'm not saying that it WILL be a retread, but people are usually not like fans. Fans will do the research, read the articles and know that they are different films. But the majority of people are not fans. They see the trailers such as they did for Wild and will do for Ratatouille....they will see Rats in the sewers. I know, and you know they are two different movies. They will not. They will see a movie that comes out in November with cute talking rats in a sewer. When it comes time, seven months later, for a movie with cute talking rats, they will say "seen it".
Wild was victim to the same thing. Two different stories, and the Wild was definately the better of the two.
Flushed Away and Ratatouille will be two different movies, and while Pixar has a good track record with me (other than that piece of crap Cars) I think Aardman does too...after all, Wallace and Gromit is a classic, from the shorts to the full length feature.
Time will tell on this one, but I think time is against Pixar on this one.