This answer is a good answer, but, as explicitely mentioned, ends up not fitting the question since it has been edited since the answer was made.
What should users be suggested to do in such a case?
It seems like downvoting would punish the poster for no good reason, which is what's making me hesitate so much.
Downvotes are appropriate for answers that no longer fit a clarified question. Take it as an object lesson in why it's risky to answer a question before it's clarified.
Downvotes also notify the owner of the answer that there is a problem with it. They may ignore it, or may take action, but either way the downvote has served its intended purpose: to separate the good answers from the not-as-good answers and let the answerer know that there are problems with their answer. Remember that downvotes go away if they choose to delete; and if they edit, downvotes can be reversed and they may attract new upvotes.
If you can edit them to meet the new question, do that, it's the best solution. A comment on the answer might also work, if you don't feel right editing.
If not I'd go ahead an flag them with a custom flag, explaining the situation. Sometimes a Not an Answer flag works, but generally you'll want a custom flag here.
Downvotes and Upvotes are a means to ordering the list of answers, not a reward or a punishment.
If an answer no longer fits a question due to editing, that is exactly the same as an answer that never fit the question - it's the current state that matters, purely for ordering the list purposes. The most useful (to that question) answer should be on top. The least useful at the bottom. This is so people looking for an answer to the question can get the most information in the least amount of time by reading top->bottom.
The community isn't venerating heroes and casting down villains - it's acting as a highly complex sort algorithm. It's not perfect, because what the 'community' thinks is most useful is not going to apply to outliers, and there's stuff like this - cult of personality and confusing upvotes with personal approval. But the overall goal is to simply sort the information, not to pass some sort of moral judgement.
Additionally; If the answerer wants to preserve his answer in the face of a radically different question, he is free to re-ask the original question himself. There is nothing wrong with this, and if his answer is good, it is what he should do.
Downvote. It's not relevant how much of a brilliant well crafted piece of prose an answer is - if it doesn't answer the question as posed, it's not a good answer. Upvoting and downvoting are not "reward" and "punishment" and have nothing to do with how good a person the answerer is, how smart they are, etc. It is solely about is the posted answer a good one for the posted question.
Don't flag - what do you want the mods to do that you can't do? Mod intervention is for clear off-topic cases; stuff like this we look at, say "Mmm-hm that looks like a corker," dismiss the flag and move on. If you want it deleted for being off-topic, vote to delete.
A downvote without an explanation is a very sad thing to see on an answer that you took an effort to make nice. However I agree that a downvote has the power to get attention.
Especially when we are talking about originally good answers that look weird now because of an edited question, the best course of action IMO would be:
This approach is also good for answers that deserve downvotes for other reasons. A naked downvote is a real heartbreaker.
