At first sight, it appears that our policy on changing question meanings would discourage editing this question, because it already has a valuable answer. But is that still true when the question is off-topic, thus displaying a banner that asks readers to "please edit the question"?
In my opinion, no, it doesn't still hold true in such a case. A closed question is one that should have no answers. (In an ideal world, it would be closed before it got answered.) It should indeed be edited to be brought on-topic (or, if it's not editable to be brought on-topic, should be deleted or have an historical lock put on it), and any no-longer-on-point answers can (and should) then be edited to reflect the change to the question, or deleted. (Ideally, edit the question so it's on-topic and so any good answers remain answers to the new question. But that won't always be possible.)