On one hand I feel like this is a silly request, it's just an achievement. But then again I also wouldn't care if they found a way to grandfather in users who redeemed codes prior to Conventioneer existing, because it's just an achievement.
Because achievements hold little importance except a little nod that you were there during a specific event or completed a specific task, I don't really put a lot of importance upon the achievement system or actively try to get new achievements.
So if they do it, that's fine, and if they don't, that's also fine. Either way they're going to ruffle some feathers and it's not going to put a damper on my own personal experience with the site.