Maybe they could do things sort of according to posts but not directly applied to number of posts like:
1-300 posts = default titles
300-600 posts = chosen titles, 1 week before you can change them
600-1000 posts = choses title, 3 days between changes
1000+ posts = chosen titles, change available every 24 hours
or make more groups or something. I dunno if this would work as there are many lurkers around the forum that don't post much but have been around a long time. i think this would work as only people that have been here and contributed to the forum can choose their own user title. Even better, put in on how many months a user has been here so it stop people spamming to get to 300 posts and change their user titles. Iono just making discussion :)