Duplicate questions are discouraged, but an answer that was up-to-the minute last year may be fairly tired this year, so to encourage people to keep answers up-to-date, particularly the more highly voted questions and answers, perhaps a badge could be awarded for a / [so many] revision[s] of at least [this length] on answers [at least so old] with at least [so many votes] that subsequently receive [so many more votes].
I'm not sure this is needed. I don't think that editing someone else's answer and changing its substance (which seems like what you're suggesting) is really even allowed. I don't think we want to encourage this behavior.
I'd rather see someone post a new answer to the old question, but we already have
The only situation this doesn't cover is revising your own answer with updated information, but I don't know that a corner case like that is really worthy of a badge, especially since you're getting all of the rep for it anyway.