This question already has an answer here:
Voting is one of the most important things on the network. An answer's score lets you know if the community trusts it; if you're likely to be getting correct information or not. And since our goal isn't just to help the OP, but to build a repository of information that helps the Make The Internet A Better Place, it's important that this voting be accurate. If downvotes linger on improved content, we're not marking it properly for future readers.
In an effort to combat this, I'd like to propose a variation on Jon Skeet's request to be notified when a post you've downvoted has been edited. I feel that the main reason for opposition was that people didn't want to be bothered by notifications every time there was an edit. So instead, I'd like to propose that we add a tab to your profile which displays edits to questions you've downvoted (as a subset of either the Votes tab).
No one would be bothered with notifications if they weren't interested in this info, and those who do want to be able to go back and see if they should reverse their votes will have any easy way to do so. Currently the only way to keep track is to keep a bunch of tabs open with posts you want to revisit. This isn't really practical, and eventually you're going to stop checking... This way you don't even have to think about it until it does happen, and you get a passive notification on your profile that you can look at whenever you like (or not at all!).
The ultimate goal here is to remove downvotes on posts that no longer warrant them. This helps us improve our system of distinguishing good vs bad content, which makes SE a better place.
Another thought I had is that this list could be filtered to only display edits made by the post owner. This would decrease the number of posts displayed (making it less overwhelming if you vote a lot) and also increase your chance of viewing an actionable post (making it more useful). I say it increases your chance of finding a post you want to change your vote on because, in general, we don't make edits to other people's answers which substantially change the meaning of the answer. So if an edit is made by the post owner, it's more likely to be an edit which would warrant changing your vote.
Anyway, these are just some thoughts. I look forward to hearing what the community has to think! This idea was sparked by comments on this meta question, where it came up that maybe a variation on this (highly voted!) feature request should be brought up for further discussion.