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[Logician]

Create a new user account for him inside Admin CP with his old username. Then edit this account's "userid" in "user" table as his old userid (you can learn his old user id by checking people's ignore lists, it should be recorded there). Then run this query:

UPDATE post SET userid=XXX WHERE username='YYY'

XXX is his userid
YYY is his username (be careful about caps!)

That will do the trick..