In Picard Beverley says that the changeling on the USS Titan is “evolved” as it can hold its form even after death and only reverts after severe dissection. However that makes little to no sense. For one that would mean they didnt need to revert to their liquid form every few hours as they dont need any effort to hold their form.

Furthermore the changeling CGI seems to show them in their liquid form as though they are made of tendons and ligaments and muscle. But that would be more characteristic of a transitional phase between solid and shape shifter. They also dont seem to change in mass, even aboard the Shrike the changeling seems to be almost the same size as the fist it was imitating and the Human versions also appear to be the same in size across their liquid and solid forms.

In conclusion the changelings didn’t evolve, the writers were just lazy and kept bringing back old villains which didnt even have anything to do with Picard to make the show interesting. This would not be a form of evolution for a changeling and evolution, especially this extreme, takes millions of years, not a few decades.