So here are three questions which discuss why changing a question once you've asked it is wrong:
- question edited to change meaning
- Can I change the existing question to a new one?
- "Bug" in reputation system: correct answer forces change in question, invalidating answer, causing downvotes.
The most common reason seems to be that existing answers are invalidated. However, I have the situation where a question I asked (edit: I've now deleted it) has no answers yet, and the comments suggest that only some of it may be answerable.
Given that I've had no answers yet, is it reasonable for me to change the title and content of a question to a more specific and concrete subquestion of the original question, which can then be properly answered?