The other hypothesis, that Black Widow’s hair has been CGI’d to hide the time jump, is very interesting. We have heard that Scarlett Johansson’s character might be going back to her signature red hair in this film, along with a shorter haircut. In theory that’s no big deal, after all, the trailers for Avengers: Infinity War didn’t hide that some of the heroes (including Black Widow) had new looks, and we’ve seen that Cap trimmed his hair and shaved off his beard in the Endgame trailer.
Article source: https://www.cinemablend.com/news/2464289/is-black-widows-hair-cgid-in-the-avengers-endgame-trailer