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Cue the Creationist / New Ager / other Believer:

"But scientists once thought that, and now they think this!"

Yes. scientists continually update their beliefs to fit the evidence; that's the whole point, the whole beauty of science. 

While no scientist has ever had trouble thus refuting the silly complaint above, I really like this quote from economist John Maynard Keynes, responding to an accuser charging him of inconsistency:

Well, when I get new information, I rethink my position. What, sir, do you do with new information?

Nicely said! 

Addendum:

Carl Sagan had words to say on the matter as well. From the 1987 CSICOP keynote address:

In science it often happens that scientists say, "You know that's a really good argument; my position is mistaken," and then they actually change their minds and you never hear that old view from them again. They really do it. It doesn't happen as often as it should, because scientists are human and change is sometimes painful. But it happens every day. I cannot recall the last time something like that happened in politics or religion.

Bingo.