I'm wondering if it is ever a good idea to edit the question to better fit the answer.
Before you reply "Absolutely Not," here is the question I am talking about:
Email client that supports Markdown
The OP asked for software that will work on OSX. Shortly after asking, another user leaves a comment and asks if he can edit the question to ask for a cross-platform solution, as he is also interested in a solution, but is using Linux. The OP replied that he did not want to edit the question, but suggested that the user ask a new question.
After that exchange, an answer was posted with a cross-platform solution, and accepted.
I realize that it is certainly possible that a Mac-only answer will come later to this question, but if some time passes with no Mac-only answers, would it be a good idea to edit the question to ask for a cross-platform solution, to prevent "duplicate" questions in the future that need an identical answer? Or is it better to leave the question and answer as is, and allow people to post another question in the future looking for a Linux or Windows solution?