Warning: SPOILERS for Star Trek: Discovery Season 4, Episode 8 - "All In"
A Changeling appeared in Star Trek: Discovery season 4, episode 8, which is the first sighting of Star Trek: Deep Space Nine's villains in the 32nd century. After Cleveland Booker (David Ajala) and Dr. Ruon Tarka (Shawn Doyle) went rogue on a personal mission to destroy the Dark Matter Anomaly, Captain Michael Burnham (Sonequa Martin-Green) tracked down a broker she and Book knew from their days traveling the galaxy as couriers. This led to the former lovers confronting each other at the poker table in a Karma Barge near the Galactic Barrier where Book and Tarka also encountered a Changeling.
Star Trek: The Original Series introduced some aliens could change their physical form, including the Chameloid (Iman) on Rura Penthe who impersonated Captain James T. Kirk (William Shatner) in Star Trek VI: The Undiscovered Country. Changelings became central to DS9 because Odo (Rene Auberjonois), the shapeshifting Chief of Security, was a main character. DS9 soon revealed Odo was part of a race of liquid-based Changelings called the Founders who ruled the Gamma Quadrant as the Dominion.However, Odo wasn't aware he was a Founder at first; instead, Odo was one of a hundred infant Changelings sent into the galaxy. The Dominion eventually sought to conquer the Alpha Quadrant and wage war against the United Federation of Planets. By siding with the Federation against the Dominion, Odo became the first Changeling to ever reject his own people, although Odo did join the Great Link at the end of Star Trek: Deep Space Nine.
In Star Trek: Discovery season 4, episode 8, "All In," Book and Tarka sought isolynium, a substance the mad scientist needs to power the isolytic subspace weapon he plans to use to destroy the DMA. Book agreed to earn his way to his prize by helping spot illegal gamblers in the Karma Barge, which led the Kwejian to suspect an alien whose blinking eyes appeared to be signaling other players at the tables. Soon, the alien realized his cover was blown and tried to escape, which is when he started shapeshifting to impersonate other aliens. Tarka quickly captured the shapeshifter, who reverted into her (presumably) true form of a female Changeling from Star Trek: Deep Space Nine. The Changeling has a lack of facial features similar to Odo and the other Changelings seen on DS9, although she adopted a hairstyle similar to the Cardassians.
It's exciting to see a Changeling on Star Trek: Discovery the shapeshifter's cameo didn't provide any information as to the state of the Dominion in the 32nd century. The Founders are able to live for centuries and it's quite possible the Dominion still exists 800 years after it lost a war with the Federation. Seeing a Changeling does beg the question of why the Dominion didn't attack the Alpha Quadrant during the century the Federation collapsed because of The Burn if the Founders are still operating their totalitarian empire. It's also unclear if the Changeling Book and Tarka caught is part of the Dominion or if, like Odo, she was one of the hundred sent into the galaxy to fend for themselves, but it's probably the latter if the Changeling spends her time cheating at gambling and rubbing shoulders with the less desirable elements of the galaxy.
Star Trek: Discovery has systematically reintroduced many alien races who were popularized on Star Trek: Deep Space Nine and the Changeling is the latest, joining the Ferengi, the Lurians, and the Trill. Meanwhile, Federation President Laira Rillak (Chelah Horsdel) is a Bajoran, Cardassian, and human hybrid. Star Trek: Prodigy recently brought the USS Protostar to the Gamma Quadrant in the post-Dominion War late 24th century but Starfleet's fastest ship of the era didn't encounter the Founders, although Odo was one of the Star Trek icons in Prodigy's Kobayashi Maru holodeck crossover. But now that Star Trek: Discovery has confirmed that Changelings are still part of the 32nd century, maybe, one day, there will be a status update on the Dominion.
Star Trek: Discovery streams Thursdays on Paramount+.