This may be more of a suggestion, but I'm not sure how you'd tackle this with the overhead of SQL queries, and if it should be done at all.
When a user posts a thread, their username at the time is saved in the post/thread table, hence :
Discussion in 'Suggestions: Bigger Features' started by vrtsolus, Jul 31, 2010.
However, if the principle post username is changed, this is not reflected in the thread.
I am wondering if this has an effect on SEO, or what could be done, or should be - if any at all.