Ok I thought HN was smart enough to re-open comments or something for old stories.
Personally I don't like this workaround. Because we now have several threads for the same article. And tools like Hacker News Sidebar [0] don't work anymore.
But granted for my original question this is probably the way to go.
If HN just redirected you to the old story, append ?resubmit=hn or something similar to the URL.
Obviously, you should only do this if the original story is old, maybe a year or more.