...That happens to be how science works- you constantly rework your model to match reality. If you want eternal consistency (well...kind of), go read a theological text.
For the record, the reason Pluto was reclassified was because the term planet wasn't really scientifically defined until 2006. Before then it was actually a pretty loose term. They discovered a tenth body that was larger than Pluto, which prompted them to put their foot down on what should be classified as a planet and what shouldn't.