What is interesting is that it appears to work for most stories regardless of the story's age and still gives karma to the original poster.
The oldest one I was able to upvote was https://news.ycombinator.com/item?id=2 (changes pictured at http://i.imgur.com/ISDYNta.png) but https://news.ycombinator.com/item?id=1 didn't work (instead of upvoting the original it was submitted anew).