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: 
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).
This seems like a highly requested feature, and still doesn't have enough ROI.  

**Why?**

1. **We like notification, not spam**. which one of those two is more likely to happen? well it depends on the quality of the feature. Users with thousands of posts may have to either spend a lot of time to manage subscriptions, or just get spam.
2. **Users should "play" fair** - changing a major thing in your post after you got upvoted answers can be rude. You can never avoid users from "abusing" a system.
3. **The core functionality is mostly achieved through other activities** i.e by the first down vote or comment.
it's clear that it's not fair, and the down voter may never return to change his vote regardless of what you do. if it's a highly ranked post, most chances that you will be know that there was a change quick enough.