Thanks for the question, Paul.
Some advances that put fossils in new light come partly from finding new specimens - these fill in gaps in time and space and evolutionary history. We'll always find new fossils that adjust our thinking about old ones.
Other advances come from new technologies, or new ways to use old technologies. CT scanning is a huge help that has changed the field since I've been born. It offers ways to look inside a skull and reconstruct the sinuses, blood and nerve pathways, and the shapes of the brain, among other things. So now we have enough of these scans that a bunch of paleontologists are studying the evolution of dinosaur brains. CT was a game changer because it allowed a whole new area of research (paleoneurology) to exist.
To answer the other part of your question: Science builds on previous ideas and evidence. It's rare that one piece of information overturns years or decades of previous evidence, but it happens. When you see a news report saying THIS CHANGES EVERYTHING WE KNEW ABOUT XYZ, that's almost always an overstatement or hype. Sometimes we do get a critical piece of evidence that puts everything else in context, or unlocks whole new areas of understanding. More often, evidence builds for a while and then later we can look back and go, man, we've come a long way in ten years.