Just my opinion but I think old usernames should remain even when they are not active for awhile.
Some people do come back after being gone for a long time, and at times for a matter of reference it's good to be able to search by username for threads by a specific user. I find myself doing this often in the DBMan forum when I find something interesting and want to view all related posts for that type of database by a specific user.
When usernames are changed all threads posted revert to the new username :(
I think it's very confusing when people keep switching their username :) You form a reputation with your username and I think rather than changing your username you should just choose a NEW one and leave the old threads in tact, so to speak.
Unoffical DBMan FAQ
http://creativecomputingweb.com/dbman/index.shtml/