I think it's best to edit the information into the answer.

One option is to add an answer with the new information. This is only good if the Question asker, changes the accepted answer. This is unlikely to happen though. So the problem is that the new (up to date) answer will be below the outdated (possibly incorrect) answer.

This means that visitors to the site coming from a search engine with the same question will first see the old accepted answer and then be given incorrect/outdated information. It's unlikely that they will go through all the answers before leaving, having been disappointed by the quality of our answers, this, I think, is bad for the site's reputation.

This is evident by this answer where a user added another answer likely because they did not find the information already in the top answer.

If new information becomes available then it should be added to an already accepted good answer. This allows for the best answer delivered to future visitors in the shortest amount of time.

The point is to give the best possible answer to questions, if we need to edit in more information then what's wrong with that?

As to the argument that it's "going against the OP's intention", I don't think that it is because they did not know about the new information at the time they posted. I would not mind people changing my answer so that it continues to be up to date and relevant. It means that the answer will provide even better information to anyone looking to answer a question.

This I think will also discourage downvoting of a good answer that has become outdated in an attempt to change visibility.

I think it's best to edit the information into the answer.

One option is to add an answer with the new information. This is only good if the Question asker, changes the accepted answer. This is unlikely to happen though. So the problem is that the new (up to date) answer will be below the outdated (possibly incorrect) answer.

This means that visitors to the site coming from a search engine with the same question will first see the old accepted answer and then be given incorrect/outdated information. It's unlikely that they will go through all the answers before leaving, having been disappointed by the quality of our answers, this, I think, is bad for the site's reputation.

This is evident by this answer where a user added another answer likely because they did not find the information already in the top answer.

If new information becomes available then it should be added to an already accepted good answer. This allows for the best answer delivered to future visitors in the shortest amount of time.

The point is to give the best possible answer to questions, if we need to edit in more information then what's wrong with that?

As to the argument that it's "going against the OP's intention", I don't think that it is because they did not know about the new information at the time they posted. I would not mind people changing my answer so that it continues to be up to date and relevant. It means that the answer will provide even better information to anyone looking to answer a question.

This I think will also discourage downvoting of a good answer that has become outdated in an attempt to change visibility.