It's the same way with Ceres--when it was first discovered it was called a planet, until eventually scientists realized there were a whole SLEW of objects in that orbit. Now we just call it the asteroid belt, and Ceres is just one of the largest asteroids (perhaps the largest? I'm unsure) in that orbit.
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The last time I looked at a science text book, more than a decade ago, Ceres was listed as the largest asteroid with a diameter of almost 500 miles. More important is the fact that Ceres is round. It's almost a perfect sphere. Other asteroids are shaped like a potato or a grain of rice or a pork chop. (No, I'm not hungry. That's just the way my mind works.) Ignore size and a few other little details and it's a planet.
Howard