Somewhat following on from this recent [meta question](https://cs.meta.stackexchange.com/questions/944/should-i-ask-a-new-question-or-edit-the-current-one), how much of a change should someone make to a question before they should simply ask a new question?

As a case study, [this](https://cs.stackexchange.com/questions/10071/how-to-find-a-local-minimum-of-a-complete-binary-tree) question was asked some months ago, then apparently abandoned by the original poster immediately after asking. A few hours ago (at the time of writing) a second person edited the question in such a way that the meaning changed significantly (from "what is this property" to "how do I compute this property") and invalidated the answer. Some things to note:

* for full disclosure the invalidated answer was mine (this was how it came to my attention)
* the original question was, in my opinion, trivial
* the new question is much more useful (again, in my opinion)

These points aside however, it still seems like a step too far to change a question (with answers, well, one not-that-interesting answer) in such a significant manner (especially after such a long fallow time).

It's obvious in the extreme case of course, where a sensible question is replaced with nonsense, but here it also seems like an evaluation that we should not be making: "this question is too trivial, and I know a better, related question, so I will *replace* it". Part of my unease may also stem from the fact that a second party is making this judgement, rather than the original author working out the real question the wanted to ask.

So what is everyone's opinion on where we should lean on this kind of editing? Should editing always preserve intent, or more pointedly, should we replace bad questions with better ones?