Question: Consider this situation. I post a legitimate and correct answer to a question and receive a few upvotes. Then the OP realizes that's not what they wanted to ask, and significantly rewrites the question, rendering my answer irrelevant and useless. Ideally the OP would comment on every answer saying "check my edit" but *some people don't know to do that*.

This leaves two possibly bad situations:

1. I never know, the answer stays, and the votes don't reflect the usefulness of the answer anymore.
2. All of a sudden I get an influx of downvotes because my answer isn't helpful anymore. *If I'm checking and notice my rep has dropped*, I come back after the fact, edit the answer, and bring it back to the original quality. This is great, but 9 times out of 10, the downvoters never come back and notice the answer has been edited. It might get more upvotes, but it still doesn't reflect the true value of the answer.

If we can get notified when the OP gets edited, we can nip it in the bud and go back to edit our answers to match - thus maintaining the quality of the answers on SO.

Answer: 
An alternative would be to show clearly that particular answers were written **before** the original question was edited, for example:

> 
> This answer is for a previous version of this question. *Click here* to
>  notify the user if their answer is no longer relevant.
> 
> 
> 

Another option is to display an alert to the downvoter when downvoting an out of date answer.

I think the majority of SO users would be glad to edit their answers if they're no longer relevant to an edited question, but without notification it's extremely difficult to check before getting downvoted.
Wrong solution because you are trying to address the wrong problem. 

Changing the question out from under good answers is simply **rude** and should be discouraged. I wrote an [expansive version of this argument](https://meta.stackexchange.com/questions/49821/when-editing-the-question-option-to-notify-all-answer-authors/49831#49831) on [a later question](https://meta.stackexchange.com/questions/49821/when-editing-the-question-option-to-notify-all-answer-authors).