If, after posting a question for some time, I found out it is another related but quite different question that really interested me. Is that OK to just modify the original question to the new one, or it's more preferable to post a new question?
If there are answers, or even comments, you shouldn't invalidate them by changing your question.
If no one responded to your post at all, perhaps delete the old one and post the new one.
In general it is preferable to post a new question.