<p>You should not make <em>substantive</em> changes to someone else's answer.  This has been discussed before on Meta.CV, e.g., in <a href="https://stats.meta.stackexchange.com/q/5368/7290">Are you allowed to edit an answer that you disagree with?</a>.  That does not mean that no edits are possible.  You can see <a href="https://stats.stackexchange.com/users?tab=Editors&amp;filter=all">here</a> that over the years I have found nearly 10k places where an edit would be beneficial (I believe edits to my own posts are not included in that count).  Many of those were answers, but none of them were substantive (excepting the instance mentioned below).  Edits are best for things like spelling, punctuation, grammar, formatting and clarification that preserves the authors intent.  The post that is listed as by a user, even if they've deleted their account, should reflect what they wanted to say.  If you think an answer is incorrect, or incomplete, you can vote down and/or leave a comment.  You are free to add your own answer and reference theirs.  But you should not make a substantive change <em>without their permission</em>.  The idea that a figure would be helpful is, at least in the abstract, quite reasonable.  I can relate <a href="https://stats.stackexchange.com/a/101490/7290">an occasion</a> in which that happened with me.  I made some plots to illustrate a point someone else was making, and I thought they would benefit the answer.  So, I asked them in comments if it would be OK.  If they had said no, or not responded because they were no longer around, I could have put them in another answer and referred back to the answer at issue, and I could have put another comment under that answer pointing to mine.  But I would not have changed someone else's answer in a meaningful way without their permission.</p>
