I would say that if your question and the posted answer remain accurate for some context, but not the latest context, keep it as the accepted answer, and edit the question to indicate the limited context. For instance, if you asked "Can I run apps so they take up the full screen" under Leopard, the answer, in general was "No, there is no generic full-screen mode." Lion comes out, and now there is. Your question and its answer are still useful for someone running Lion, so edit the question to indicate "prior to OS X 10.7, can I run apps so they take up the full screen."
For released, versioned software, this strikes me as far and away the most appropriate way to go. On the other hand, in your case, the software you describe is an online service; the fact that something worked or did not work in previous versions is sadly not helpful, as the current behavior of the system applies to all users. In this case, the answer is no longer correct. Were it me, I would not un-accept until a more current answer arrives to replace it, but I would point out in a comment the changing circumstances, and perhaps edit the original question also.