Suppose that while browsing through old questions, I find that the top answer, while almost certainly correct at the time it was posted, is now incorrect. I add a new answer with the current, correct information. Should the old answer be downvoted?
Just as it's appropriate to change the accepted answer to your question that was once correct, it is appropriate to downvote a once-good answer.
Before I do this, I would first comment to why the answer is no longer correct, perhaps with an errata link or some other damning source so that they may be able to edit the answer themselves.
You can vote however you want.
That said, saying that all the site's users should also cast their votes a certain way in certain situations is an overreach. Users aren't ever under any obligation to vote one way or another or even to vote at all. If new information has rendered an answer incorrect, it's up to each user to determine if that answer now deserves further action. A mandate or policy or even a perceived consensus that certain classes of answers should be downvoted all the time no matter what violates users' voting independence, something the site greatly values and respects.
To be clear, if you downvote an answer, that's on you, and you should be acting here according to your own will—that is appropriate. If there's a desire to see users downvote an answer, that can be encouraged, but I suggest that be done with the tools the site already provides—put a bounty the question, add appropriate comments, and cast votes according to conscience—rather than, for instance, a comment on the obsolete answer that says This answer is incorrect in light of rules changes; for why it's been downvoted see this Meta question. It's better for voters to downvote and leave no comment than to have voters downvote and in comments hide behind a bureaucratic shield that absolves them of the responsibility for their votes.