I have never seen a set answer or reference from GN about this either, but would be interested in knowing what they think would be best.

I have done, and have seen it done, a couple of different ways, all of which worked: 1) an update added to an existing story... ie: put an 'update' subheading and the update under the main article; 2) an 'update' subheading added with brief information - then a link to an updated story if the update warrants a whole 'nother story; 3: including 'live updates' in the title and updating the story multiple times throughout the event (I've seen this done successfully over at bleacherreport). I have also added a correction notice, with a link to the 'update' below the main article, near the top of the story, when an important correction is needed (a rare occurrence, but it happens on breaking stories).

Regardless, GN revisits new pages often, so they will pick up changes/updates to the content. As the story ages, they do not revisit as often. Some details on this from GN are here: http://support.google.com/news/publisher/bin/answer.py?hl=en&answer=40392&topic=2492117&ctx=topic