Generally you can get by without apologizing to online communities if you just suddenly start acting appropriately and maintain it. Barring a few famous problem users, nobody is keeping a tally of how much they dislike you. The attention span of the forums is way too short.
For instance, I registered this account when I was ten years old, and naturally all of my old posts are exactly what you'd expect from a ten year old. But since my post quality is higher now, I've never had anyone seriously try to criticize me for anything dumb I might have written back in middle school.
Making an apology thread might even have the opposite effect, because now I'm aware of the fact that you've given people reasons to dislike you in the past. If you hadn't posted this, I would have no idea who you were.