[ConqSoft]

Something like this. Though, this hasn't been tested, and was just typed off the top of my head.

UPDATE adv_gallery_images SET userid = (SELECT user.userid FROM user WHERE adv_gallery_images.username = user.username)

and

UPDATE adv_gallery_posts SET userid = (SELECT user.userid FROM user WHERE adv_gallery_posts.username = user.username)


And if you have any user-owned/created categories in Coppermine, you'd need to run a similar update on the Gallery category table.