Well, that's exactly what I want to know. This is just didn't make sense to me about the ending of EP31 of KR decade.
Basically, that preview was made for the movie Kamen Rider Decade: Decade vs All Riders.
The initial announcement for the December movie in 2009 made it seem like we'd get a Rider/Sentai-like double feature, with two unrelated movies. There was Kamen Rider Decade: Decade vs All Riders and Kamen Rider W: Episode Zero.
However, there were heavy production changes and we ended up getting something very different in the end, with the two stories tying into each other directly and going into a third part. As far as the Decade part itself goes, in addition to that change in structure, the movie also had to be rewritten, because, like the title clearly shows, it was originally intended to be another All Riders, rather than just something using Decade's cast.
The Decade preview that you saw was recorded alongside the tv show, so those scenes were never actually intended to be in the movie, but the movie diverged from them even more than expected due to that heavy change in structure. However, even if those changes hadn't happened, we probably wouldn't have seen those scenes exactly like they were in the preview, the movie just would have been closer to it in some ways.