Execute these with Query Analizer:
exec sp_changeobjectowner 'old-owner.FORUM_BADWORDS' , 'new-owner'
exec sp_changeobjectowner 'old-owner.FORUM_GROUP_NAMES' , 'new-owner'
exec sp_changeobjectowner 'old-owner.FORUM_GROUPS' , 'new-owner'
exec sp_changeobjectowner 'old-owner.FORUM_MEMBERS_PENDING' , 'new-owner'
exec sp_changeobjectowner 'old-owner.FORUM_NAMEFILTER' , 'new-owner'
Thanks to Gremlin for this, since he posted about it at the moderators forum.