Black Widow's hairstyles in the Avengers: Endgame trailer could be a clue about a possible time jump. The trailer hasn't even been out for a full week and Marvel Cinematic Universe fans have been going through it and finding all kinds of little details that could lead to bigger implications on the storyline. Time travel and a time jump have been discussed in fan circles ever since some set photos found their way online, showing Tony Stark looking a little older along with shots of the Battle of New York from the first Avengers movie with Scott Lang involved.
The Avengers: Endgame trailer makes it seem like the sequel takes place directly following the events of Infinity War. However, there are a few clues that point to a possible time jump and time travel as well, possibly later on in the movie. A clever Reddit user noticed that Black Widow has the same hairstyle as she did in the last movie while talking to Steve Rogers and noticing that Scott Lang is on the security camera footage. Later on when she finds Ronin, her hair is clearly longer and pulled back, which leads one to believe that there has to be some kind of time jump involved. Additionally, Black Widow is back to her red air in the official concept art for the movie.
Ronin is another key to there possibly being a time jump in Avengers: Endgame as well. It has not been confirmed, but many believe that Clint Barton's family was wiped out by Thanos' snap, which is why he becomes Ronin. If that's true, it would probably take some time before he pulls his act together and takes on the Ronin identity. Again, this is just speculation, but it starts to make a little bit more sense when looking at some of the set photos as well and then seeing Scott Lang on the archived surveillance video at the New York Avengers headquarters.
The Quantum Realm will factor into Avengers 4 and it will explain how Scott Lang escapes and ends up at the Avengers headquarters. Was he able to get out in a different year then he went in, or is he in present day? It's unclear at this time, but it is clear that he was safe from Thanos' snap while inside the mysterious Quantum Realm. Hank Pym's technology could be what is pictured on the wrists of our heroes in the leaked set images as well, leading one to believe that the time travel is all but guaranteed at this point in time.
With all of this information, nothing has been confirmed by Marvel Studios at this time, and out of context photos from the set of Avengers: Endgame don't really prove anything on their own. It's also entirely possible that Black Widow is still in hiding and could be using a wig to further disguise herself. But, a time jump does make a lot of sense with the current information that we have. This Avengers: Endgame fan theory originated from Reddit.