[Stu]

I did something like this on a test board,
I ran queries on the Database to change the 'username' and the 'userid' within the 'Post' table
I'm sure this was all I did, but to be sure I would advise you to try this on a test board first, or wait for a more experienced member to offer a solution

UPDATE post SET userid=newid WHERE userid=oldid

UPDATE post SET username='newname' WHERE username='oldname'

then updated the counters from adminCP

Depending on what degree it may be necessary to run these too..

UPDATE thread SET postuserid=newid WHERE postuserid=oldid
UPDATE privatemessage SET fromuserid=newid WHERE fromuserid=oldid
UPDATE privatemessage SET userid=newid WHERE fromuserid=oldid AND folderid='-1'
UPDATE subscribethread SET userid=newid WHERE userid=oldid
UPDATE subscribeforum SET userid=newid WHERE userid=oldid
UPDATE pollvote SET userid=newid WHERE userid=oldid
UPDATE calendar_events SET userid=newid WHERE userid=oldid