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find all the tables that have a CreatorID column:

  SELECT DISTINCT TABLE_NAME 
    FROM INFORMATION_SCHEMA.COLUMNS
    WHERE COLUMN_NAME IN ('CreatorId')
        AND TABLE_SCHEMA='opensim';

=> yields assets inventoryitems prims primitems
as tables matching the column.

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replace all the creator ids with fred's id on ducky:

  update inventoryitems set creatorid = 'NEWGUID'

=> do for each of the tables.

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checking to make sure the changes to creatorid took effect:

  select * from assets where creatorid != 'YOURGUID'

=> there should be no matches after running a setting operation.

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if you have a more selective update to do, try replacing using a pattern:

  update assets set creatorid = replace(creatorid, 'OLDGUID', 'NEWGUID')

==============

note: replacing all creator ids like above still does not secure total
ownership for everything in inventory or even in the sim; nested objects in
inventory do not seem to be affected, and the contents of objects nested in
live objects also are not affected.