Here is (or was?) my understanding on how things are supposed to work: when editing, the key rule is don't change the main point of the question. So whenever you want to make an edit, think about whether it would change the core of what the question is asking. If it would, then you should make a new question instead of editing.
Note that this has nothing to do with whether you might invalidate the existing answers; you're not obligated to make your question conform to the answers that are posted. What happens reasonably often is that someone reads your question, doesn't correctly understand what you were asking (either because your question was unclear or they just misinterpreted it), and posts an answer that doesn't actually give you the information you wanted. In that case it's perfectly fine and highly encouraged to edit the question to make it clear what you were looking for and why the existing answer doesn't cut it. As always, the key is that you're clarifying the question, not changing what it's asking.
When this happens, it's really best if you stay on top of things and make the edits as soon as possible after the misinterpreting answer is posted, so that the poster of the answer can edit it, and so that future answerers won't do the same thing. If you let the question sit around with an inaccurate answer for more than a couple days, people are less likely to come back to it after you edit. And in that case, it might just be better to edit the question to ask what the answer answers, or perhaps request to have the question deleted (but we sort of shy away from that), and then post a new question with the clearer version of what you originally intended to ask.