<p>The third solution--commenting on the answer--is probably the better choice. If, say, this is just a matter of altering initial conditions or hypotheses to see what happens, the answerer could easily reply in a comment (for a short response) or as an edit to his reply (for a longer discussion). </p>

<p>If existing response(s) are suggestive of a lack of information or precision in your question, update your question; answerers will be able to update their responses as well.</p>

<p>In case the question is really altered, it should be better to start a new thread with proper reference to the original one, but see <a href="https://stats.meta.stackexchange.com/q/1151/930">What to do when the original question is transformed considerably after getting clarification?</a> I do not think this applies in the present case.</p>
