That's more a function of us not having the current definition of what a planet is growing up than anything.
Pluto got "downsized," because we had to refine the definition of what is and isn't a planet. Not our fault, and also not the fault of the previous scientists.
Kinda the one thing that I hate about the public's understanding of science. Most people do not grok the concept that, "no one can ever prove a theory correct, but one person can prove that, at least in specific circumstances, it is wrong."