This would be a person making an edit to that's person's own prior answer that makes a significant improvement. Say it's an algorithm improvement that reduces time complexity (such as from O(log(n)) to O(1)), but that the answer was already accepted and/or upvoted.
I'm thinking that for significant improvement, for the benefit of others reading the asnwer, the improvement should be inserted at the front of the answer, noting the change and using a separator to retain the original answer after the inserted improvement.
I'm updating this question with a specific example, an answer about a Visual Studio 2015 change to std::list::sort. The improvement in this case was a switch from directly accessing nodes (std::list::splice does this), to using iterators. Rather than replace the prior answer, the updated answer has the improved version at the start of the answer, with the prior answer at the end. I left the prior answer there because of comments. However, the prior answer could be considered obsolete, and I'm wondering if I should just remove it, and leave a brief description of the prior answer so that the comments would still make sense.