Update Trigger in MYSQL
Unfortunately you can't use a trigger to update the table that called the trigger: from the documentation: 


  A stored function or trigger cannot modify a table that is already
  being used (for reading or writing) by the statement that invoked the
  function or trigger.


Could you move the admin field to a separate table which just holds the current admin id (or list of), and then you could update that with the trigger.

(of course if your admin is always id:1000, then you probably don't need the trigger to start with)

You would also need to consider whether to fire this AFTER INSERT and/or UPDATE.

An example trigger would be:

DELIMITER $$
CREATE TRIGGER update_user_admin
AFTER UPDATE ON users
FOR EACH ROW
BEGIN
    IF new.id = 1000 THEN
        UPDATE userAdmin SET id = new.id;
    END IF;
END;
$$
DELIMITER ;