From this threadIn reply to:
This hacks allows you to:
a) Change a username to something new
b) if the new username already exists, the script will merge the usernames.
All the settings from the old username will be kept - except for the total posts number, which will reflect the correct totals. The title doesn't get reset though, so you may need to modify it manually.
PMs sent and received, address book entries, moderators, subscriptions, read receipts etc etc are all handled and reflect the new username.
I built on Dave's great work - thanks!
Max
I only changed a couple lines to get it to work. I also had to add in a section to change the "last post by" section, because it wasn't changing the name there.