Well, I see it more as the direct predecessor in that I tend to see the events of Dissapearance as showing the effect E8 had on Yuki.
At the crux of it, she was designed to watch over Haruhi for a lifetime; however due to the events of Endless Eight, her personality, programming or however you wish to percieve it, was subjected to many hundreds of years of reality; which in essense gave her time to grow, or as her original design would see it, bug.
That's why I see it as the direct predecessor; I personally see those events as the events which directly gave her the time to develop; and being the only member of the group who actually remembered every SINGLE one of the thousands of loops, I find it impossible not to believe that could have been the trigger in some form or another; she simply wasn't meant to have been running for that long, nor did her creator forsee every possibility that could happen, as was shown by Asakura.
That's my personal opinion on it; I felt, from the show, she changed slightly after E8; and Dissapearance was the continuation of this unexpected evolution, and in its own sense, so was the scene at the end of the credits of the film.