Sometimes you give an answer and it is accepted and / or upvoted.
After some time, and growing insight, you feel you can make important improvements to some of those answers. However, the edit may include drastical changes, making it in a way quite different from the answer that was accepted or upvoted in the first place.
Writing a second answer is kind of "ugly", especially when you consider the last (version of) the answer a better one, and the second answer could be suspected to be another version of the first one.
So: what is the general idea on making extensive changes to already accepted and / or upvoted answers?