I have had a number of cases where I have voted on an answer with the best of intentions, and then gone on to read other answers and/or comments and realize that my vote was ill-cast. However, usually the realization comes too late to retract my vote.
It is really necessary to have such a short window of time to change your vote. Can it not be 30 minutes or an hour as long as I don't have a competing answer on the question? I assume the time limit is only to control/limit tactical-voting.