Is there a policy about changing the question after a period of time has passed since asking it and when there are already quality answers?
Once a question has an answer that is considered valuable by either the community (through upvotes) or the asker (through acceptance), no one, including the asker, ought to edit the question in a way that changes its meaning sufficiently to invalidate the existing answer. If people want to get answers to a different question, they should ask that question separately.
Otherwise, the edit invalidates valuable content and/or requires more work from answerers or the community to update the answers to keep up with the updated question.
In cases where a question is ambiguous, such that there are multiple possible interpretations of what it is asking for, it should be closed until it's edited to be sufficiently precise. The earlier this happens in the question's life-cycle, the better. If the asker notices that there are answers that either don't address the asker's intent or assume multiple interpretations of that intent, the asker should edit the question as soon as possible to be more precise and should comment to the answerers accordingly.