I think the reason the changeling goo is different (less liquid gold like in DS9 and more like raw meat) is because the rogue changelings are hybrids. They somehow merged their changeling DNA with the DNA of "solids". It would explain why the goo looks more solid now. I think this episode might explain why they did this: to become immune to blood screenings so that they could better infiltrate the Federation and exact their revenge.
Or perhaps the DNA of one particular soild, Jack - and now somethings gone wrong, and they need him, or more of his DNA, to fix it.