I understand the reasons for decreasing the time window on being able to reverse a vote, but I think it ultimately is going to cause more problems than it solves. I addressed this in a comment to one of Jeff's responses to another question on "tactical downvoting", but I think it warrants further discussion.
There are legitimate times when a downvote is warranted based on initial impressions or original content. However, after after additional research (or the question being edited) the downvote is not warranted anymore. By reducing the window it is no longer possible to reverse a warranted downvote for these type of legitimate reasons.
A perfect example of this can be seen by the revision history for this question. Based on the original content of the question, it absolutely warranted a downvote. However, since the question was edited, the downvote is no longer warranted. Unfortunately, having downvoted it initially, I can no longer change my vote (either by removing the downvote or upvoting).
Just to clarify the sequence of events in this particular case, they are:
- Original question asked.
- I downvoted the question.
- I left a comment explaining why I downvoted.
- Someone else downvoted the question (resulting in a -2 vote score).
- Someone other than the questioner or myself edited the question.
- I revisted the question and felt that the downvote was no longer warranted.
- I removed my comment.
- I attempted to undo my downvote and recieved the message telling me I was outside the time limit window. (I tried to undo by clicking the downvote again and also by clicking the upvote, both resulted in the same message.)
- I edited the question.
- I was then able to undo my downvote.
Based on my understanding of how this is supposed to work, I should have been successful in step 8 since it was after an edit. I also did not provide an answer to the question so I don't see how any of this could have been construed as trying to game the system to increase rep. In fact, I'm not sure how I see voting/undoing votes on a question can game the system at all; doing this on answers, yes but not on questions.