There are questions that receive huge traffic, because people need the answers. This site rocks in SEO but fails to answer these questions. Should we update questions like this one, or just let them rot?
I think it is definitely okay to edit the answers to correct dead links, grammar, and other sorts of issues.
I don't think it is okay to edit an answer an expand significantly upon it, adding new information. That's making an end-run around the fact that the question is closed and shouldn't be accepting new answers.