I haven't replied to any thread on Slashdot since I last screamed "Fuck Beta" as often as I could over a year ago. I believe I logged in once when Slashdot said they had removed Beta. (It didn't look like it to me.) I'll occasionally look at a story if something looks interesting on my RSS feed. I don't come to the website except to log in. It's been a long time since I've actually visited.
Want to show a really good faith move to attract the "old farts" back? When Soylent News was formed, they took the old Slashdot code and saw it hadn't been updated in years, and was severely broken.
Open source it again. Put a nice link on the website in an easy to find location. As a matter of fact, make it a front news story once you've done it. If you think that this is a bad move because you need to keep the code close to your chest so you don't have another split off like Soylent News, then you don't understand the "old farts" and they are not your target audience.
And even after you've done that, it will still take time for the old farts to trust you. Trust is not earned with a single action nor in a short period of time.