So today I came across a question about booming blade, which has been closed as a duplicate of this question.

They are duplicates, but a rule change has potentially made the old answer obsolete.

A bounty has been placed on the old question to get an updated answer. In this case we are lucky that the original question was asked by an active user, who is in a position to update the accepted answer accordingly (if they wish), but what if they weren't active (or don't care)?

The accepted answer would continue to be highly upvoted, marked as accepted and incorrect.

A passer by to the site won't necessarily understand how we treat accepted answers (and the general rule about looking at votes doesn't work well with older questions and newer answers), so we are directing people to what they will probably not notice as incorrect information.

What is the correct action in this situation? We need to ensure that casual viewers, which I understand are by far the bulk of users here, can read a question and come away with the correct answer (at least that is what I assume is the point of the site!).