I see your point, but the incident with Jeong Jeong brings to light Katara's healing abilities, Zhao's motivations and tendencies, and Aang's aversion to firebending. Each of these has an impact on the ending of each season.
That doesn't mean those points will be missing; it just means the films will find different ways of establishing them. That's just part of how adaptation works.
I just do not like it. I am still going to see the movie and I probably will enjoy it, but being a living Avatar encyclopedia it annoys me that such an important event in the journey is moved to the last film.
As a rule, the best way to think of an adaptation is as a separate creation that's inspired by the original, rather than demanding that it be a copy of the original. If you want something that's just like the original, then watch the original. The whole point of an adaptation is to find a different take on the subject matter. That doesn't take anything away from the original version of the story, because that version still exists. And if done well, the changes and different approaches taken by an adaptation can enhance the original work by offering new angles on its ideas and characters.
Mind you, I'm very far from convinced at this point that Shyamalan has the ability to pull this off or to make the right creative choices. But the mere fact that things are being done differently is not wrong in and of itself. The specific changes being made here could turn out badly or they could turn out very well. We'll just have to wait and see.