This has been discussed at great length. The problem this solves is that of fraudulent/gaming votes.
The basic concept behind voting now is: Be sure you want to vote, and how you want to vote, when you vote. It's a few seconds in case you accidentally click the button, not in case you re-read the answer and change your mind.
As tvanfosson mentions, there is a special case workaround for this: If you have enough rep, edit the post, then you can change your vote. There are potentially ethical issues around that, IMHO, but it's a viable option. (You could always make a trivial edit -- punctuation, capitalization, something small that poses no ethical issue.)