However, I think it would be polite for a new answer to acknowledge an old one: "Although Betty correctly says that X is not known, the following new developments shed some additional light:... ". This is important so that viewer doesn't need to try to sort out what happened.

Just washing my hands off, in case someone finds it offensive. "Necro" is already used here, and in fact the badge was given to Bjorn Poonen.

Related: if it turns out that you accepted an answer that has a fatal flaw in it, you should unaccept it and leave a comment explaining the flaw (if there isn't one already). If there's something worse than somebody unaccepting your answer, it's somebody leaving your wrong answer accepted.

You ask a question, some responses come. After some time the excitement dies down. Then you select the answer which you felt was the best.

Weeks later, after everything was forgotten, somehow the question gets bumped to the top(one of the answers getting edited, etc.). This way or by looking at somebody's profile, or whatever other way, some newcomer sees the question, and gives a really good answer. Which you feel is better than all the existing ones.

Now you are in a dilemma, whether to accept this new answer or not. What do you do, when it is considered impolite to change the accepted answer?