Use the following MySQL command to globally change all tasks not done:
UPDATE `tasks`
SET `deadline` = '2009-08-01'
WHERE `deadline` <= '2009-08-01'
AND `status` <> 'done'

Technical notes, and other ideas.
Use the following MySQL command to globally change all tasks not done:
UPDATE `tasks`
SET `deadline` = '2009-08-01'
WHERE `deadline` <= '2009-08-01'
AND `status` <> 'done'