[ozzy47]

You may be able to do the following:
Manually add a new user, then to add the old posts to that user, run this query:

UPDATE post SET userid='xxx' WHERE username='Old Name';

...with 'xxx' being the userid number of the new account and 'Old Name' being the user name on those posts.

After that, rebuild the thread and forum counters in that order:

Admin CP -> Maintenance -> Update Counters