Despite being the same race, there are many differences between Odo (René Auberjonois) from Star Trek: Deep Space Nine and the Changeling villains in Star Trek: Picard season 3. In deciding to use the Changelings as season 3's main villain, Picard brought back one of classic Star Trek's most important races, updating their history and lore in the process. First introduced in Deep Space Nine, the Changeling race has a long and complex history in the franchise, mainly explored through the character of Odo.
Odo was part of the main cast of Star Trek: Deep Space Nine and he discovered he was a Changeling at the beginning of season 3. For much of the series, Odo was at odds with his fellow Changelings because of the Dominion War but was eventually instrumental in ending the war and returning to his people. Since the end of Deep Space Nine, the fate of the Changelings has been mostly unknown, but Star Trek: Picard season 3 revealed that there had been a schism within their society and that a rogue group including the season's villain Vadic (Amanda Plummer) had left in order to mount another attack on the Federation.
How Odo & DS9's Founders Are Different From Vadic & Picard's Changelings
Based on the new information in Star Trek: Picard season 3, Odo and the Founders from Star Trek: Deep Space Nine are different from Vadic and her rogue Changeling faction in almost every way, including both physical make-up and guiding beliefs. Physically, Picard's Changeling villains are an evolved version of the species, due to experiments conducted on them by Section 31's Project Proteus during the Dominion War. With enhanced shape-shifting abilities, including the new ability to replicate humanoid cellular biology, these Changelings can now almost seamlessly as "solids", something Odo and the rest of the Changeling population are presumably still incapable of doing.
Additionally, while Star Trek: Deep Space Nine's Founders may have previously held similar attitudes toward solids as Star Trek: Picard's Changelings, their opinions since the Dominion War have become quite different. As part of the treaty that ended the war, Odo agreed to return to his people and work with the Founders to help them build a more peaceful society with less hatred towards solids. At one time, the opinions and actions of Picard's Changelings would likely have been sanctioned by the Founders, but Picard season 3 revealed that due to Odo's guiding hand, the rest of the Changeling population is now at odds with them.
Why Odo Didn't Return In Star Trek: Picard Season 3
With the importance of the Changelings in Star Trek: Picard season 3, Odo appearing in the season would have made perfect sense. However, René Auberjonois ed away in 2019, leaving a massive hole in the Star Trek: Deep Space Nine cast and effectively ending any chance of seeing Odo in live-action again in the franchise. While Picard season 3 referenced Odo, anything beyond this was impossible given that Auberjonois has ed. No official word has been given on whether Odo could be recast sometime in the future, but the fact that Picard season 3 chose not to do so out of respect for Auberjonois makes the possibility very unlikely.
Without Odo, Star Trek no longer has its most popular Changeling character. However, Star Trek: Picard demonstrated that the franchise is still capable of telling interesting Changeling stories even without Odo being directly involved. Hopefully, this means that there are still more Changeling storylines to tell after Picard, and that Star Trek: Deep Space Nine's iconic villains won't be left out of future projects.