There's quite a few open issues here about taking back a (down)vote, but I haven't found one which would fit this situation.
1) There's the time limit to take back any vote. Makes sense, I guess; abusers and stuff.
2) You can change a vote after the answer is changed. Cool!
But there's one more scenario not taken into consideration here. Have a look:
- user A posts an answer
- user B downvotes and gives the reason in a comment
- user A comes back after an hour and critiques the reason for downvote
- a discussion arises
- user B is convinced and want to take back the downvote
Then the problem arises: user B cannot take back the vote since the answer itself didn't change (only B's point of view), but there's a clear reason that B should be able to.
Having to have the answer edited just to take a downvote back is a discomfort for both A and B.
Therefore I'd like a third rule to be added to the first two:
3) You can change a vote after someone else posts a comment.
or at the VERY least
3) You can change a vote after the author comments.