When a question changes, any answers that do not answer the new version of the question should be changed as well.
There's a very simple reason for this: The purpose of this site is to provide useful answers to questions. If an answer does not fully address the question as it is posed at the top of the page, that answer is not a useful answer to that question. Our voting system functions by means of us upvoting useful answers, and downvoting (or even deleting) answers that address the question poorly or not at all.
If a question is edited after an answer has already been posted in a way that means that answer no longer fully addresses the question (and the edit to the answer is unlikely to be reverted), that answer ceases to be a useful one - and like any non-useful answer, it should be downvoted, edited to be more useful, or (in extreme cases) deleted entirely.
This may sound harsh for those users who post an answers in good faith only to have the rug pulled from under them, but there's no real alternative. Answers that don't address the question need to be downvoted or deleted, or we'll no longer be able to rely on the most-upvoted answers being the most useful.
As far as I'm aware, there are two issues that potentially complicate this question, but I don't think either of them is persuasive enough to make us give up the advantages of our voting system.
- The first is that an edit to a question doesn't allow you to reverse your upvote on answers to that question. I'm not really sure why this is the case, but I suspect it's just something we have to live with.
- The second issue is that potentially a questions can edited in ways that don't just refine it, but change it completely. As in, questions that get changed from "what are the rules for dice" to "how do I kick my mother out of the game." Such changes automatically completely invalidate every answer they have. When this happens, our response should depend on the situation.
- If the question was changed because the original wasn't a good fit for the site, answers to that question aren't appropriate either, and should be removed or changed.
- If the question was changed because the original poster decided that now they have their answer they don't need the old question any more, that user doesn't understand that questions are supposed to be persistent on this site. The user should be educated, a new question posted, and the edit to the question reverted.
- If the question was changed because the OP has refined what they wanted to ask as a result of feedback on their question... That's a grey area, and we'll have to deal with it on a case-by-case basis. In some cases it'll be appropriate to revert the change and ask a new question; In others, to let the edit stand and update or downvote and delete old answers.
We need to accept that if an answer isn't useful, it isn't useful. It doesn't matter that it was useful once, or that the person who wrote it did so in good faith; If the answer doesn't address the question, that answer isn't a good one. Yes, it's unfair. But it's the only way to maintain this site's greatness.
...On a slight tangent, when an answer is completely invalidated by a question change, it might be better to delete the answer and write a new one than to edit an existing answer into a completely new form, since this prevents votes from before the question edit sticking around.