I've seen several threads recently talking about trolling, plus ones talking about security and risk of being outed etc. They tend to have opposing views of whether allowing frequent name changes is a good or bad thing.
How about having a 'member since' and 'no of posts' tag next to the user name? Or if that is too identifying, just a newbie/long timer type rating. Some forums I've seen have a star system based on number of posts.