What does Michael Burnham's drastically different hairstyle in the Star Trek: Discovery season 3 trailer signify? When Star Trek fans last saw the good ship Discovery, she was blasting centuries into the future, preventing the rise of a deadly artificial intelligence in the present. As seen in the trailers for Star Trek: Discovery season 3, Burnham and the gang soon find that the Federation is depleted and Starfleet virtually non-existent. An event known as "The Burn" has shifted the fate of the galaxy, leaving Federation planets divided and hopeless. The Discovery crew team up with the the few remaining allies of the Federation, including, Cleveland "Book" Booker, to try and course-correct history.
Among the various curiosities in the Star Trek: Discovery season 3 trailer footage are new Vulcan characters, a Trill symbiont pool and, potentially, an evolved member of Saru's race. However, fans also would've noticed Burnham's wildly varying looks throughout the trailer. In some scenes, Sonequa Martin-Green is sporting the same hairdo she was last seen with in Star Trek: Discovery season 2, but elsewhere, she has extremely long, virtually waist-length braids. It's a vastly different look for Burnham, but there's more to the change than merely keeping up with the trends of the 32nd century.
On a practical level, Burnham's new hair indicates a time jump in Star Trek: Discovery season 3. While the trailers obviously show footage out of sequence, a logical series of events can be plotted out using the pair of promos released so far. It appears that Burnham and the Discovery encounter problems upon entry to the future, separating the Red Angel from her ship. Burnham lands alone and meets Book shortly after, who explains the 32nd century's problems and The Burn. Book introduces Burnham to what's left of the Federation, and Burnham spends approximately a year seeking answers, still without making visual contact with the Discovery. As glimpsed in the first Star Trek: Discovery season 3 trailer, Burnham grows her hair over this period, making occasional video logs along the way.
Finally, Burnham reunites with her crew, and this moment is potentially shown in the second trailer, with an emotional Tilly hugging Burnham. There are no trailer scenes of the short-haired Burnham with any of the Discovery crew, suggesting that she and Book eventually join the rest of the cast after some time alone, coming together to sort out the mess Starfleet finds itself in. Burnham's growing hair is a simple way of visually indicating the passage of time, and ensures Star Trek: Discovery season 3 won't spent multiple episodes allowing each character to adjust to their new surroundings. That transition phase will be skipped. A time jump would also allow a romantic relationship between Burnham and Book to grow naturally, therefore feeling more authentic compared to the character's previous tryst with Ash Tyler.
However, Burnham's hairstyle change serves a more meaningful function relating to the growth of Martin-Green's protagonist. When Burnham was first introduced in Star Trek: Discovery season 1, she was torn between her human biology and her Vulcan upbringing, and possessed the stereotypical Vulcan traits of stoicism and rigidness, as well as a reluctance to show emotion. This was visually reflected in Burnham's hair, which began as a short and functional affair. Granted, Burnham's style wasn't as mathematically precise as Spock's or Sarek's, but it showed a characteristic Vulcan restraint. In season 2, Burnham changed her look to a upward, curly style that expressed more individuality. This mirrored Burnham's evolving persona - her increased warmth and the the strong bonds being made among her crewmates. As Burnham's character became less Vulcan, her hair followed suit.
Burnham's journey to the fringe of the Star Trek timeline comes full circle in Discovery season 3. The long braids signify how Burnham's Vulcan-isms have almost entirely evaporated, especially after spending an extended period in the future away from her adopted Vulcan family. In the most recent Star Trek: Discovery trailer, Burnham is seen laughing, jumping for joy and expressing more emotions than ever before, and that process of self-discovery is there for all to see in her hairstyle.