The movie Arrival is hardly the first science fiction story where the writer(s) took Sapir-Whorf off in a direction that would be considered magic by some (I prefer "higher order reality"). The most famous would be the Robert Heinlein classic Stranger in a Strange Land.
In Arrival, the writers have imagined a language that so alters a
sentient being's thinking that their perception of time becomes
non-linear - future, past, and causality, all things that linear time
beings depend on, can become fluid or even meaningless.
In this specific instance, cause and effect become a chicken-and-egg problem. Did future-Chang tell future-Louise what she needed to save the past, or did he merely discuss something that happened in his past, knowing that the act of discussing it would give past-Louise information she would need?
This begs the question as to what temporal rules are in play for this story. Some SF has fluid time, where changes made in the past can have far reaching effects in the future (the butterfly effect). Some SF has resistive/reparative/contained time, where the universe has mechanisms for preventing, "repairing", or containing changes to the time line. Depending on the level of resistance/repair/containment that takes place, the high-level details of history (elected presidents, eg.) may appear unchanged to an outside observer. Some SF has rigid time, where there is exactly one time line, and it cannot be changed - the effects of traveling through time are part of the "already" established time line.
Just to confuse the issue, there is also SF that uses parallel universe travel as a mechanism to get around rigid time. You can't time travel in your own universe, but you can travel to an exact copy of your universe as it existed in the past (Time Trax, Source Code). Since the future, in that universe, has not yet happened, "time travelers" are free to rewrite it.
It is not clear whether or not the filmmakers considered this question, but I suspect that they wrote the script with either a deliberate or subconscious choice in mind. It seems most likely that they were using either rigid time or time with a strong ability to resist/repair/contain changes. I base this on two things.
One is an aside/bit of narration that Louise makes near the end:
Despite knowing the journey... and where it leads... I embrace it...
and I welcome every moment of it. (IMDB)
She knows that her daughter will die at a young age of an incurable disease, and she accepts that nothing she does will change that.
The other is something that the aliens tell her in response to why they are giving humanity the knowledge of their language - that they will need humanity's help in about three thousand years. It seems unlikely that the aliens would invest their time, resources, and even the life of one of their race without a certain amount of confidence that this investment would produce the result they needed.
So, back to the chicken-and-egg problem. Because I am a linear time being, I want to see a clear causation, and sometimes, I want to say that future-Chang was affected by learning the heptapod language and is responsible.
And sometimes, I don't think that works and have to go with an outside influence. The only agents that are outside of the system and have the resources to plant the key idea would be the heptapods. How they could have done that, I don't know, but that seems like the most reasonable alternative to giving Chang the credit.
Either way, I don't think that Louise from any part of the time line is responsible. That is why she was genuinely surprised when Chang approached her at the party.