[StefanT]

Probiere mal dieses MySQL-Query: (ungetestet, bitte vorher Backup erstellen)

Code:

UPDATE mybb_users u SET u.regdate=(SELECT p.dateline FROM mybb_posts p WHERE p.uid=u.uid ORDER BY dateline ASC LIMIT 1) WHERE (SELECT p.dateline FROM mybb_posts p WHERE p.uid=u.uid ORDER BY dateline ASC LIMIT 1)<u.regdate