UPDATE tabla SET campo3 = campo3 - campo2, campo2 = campo2 - campo1

DELIMITER //
 
DROP PROCEDURE `changeVal`
CREATE PROCEDURE `changeVal`()
 
BEGIN
	DECLARE index INT DEFAULT 2;
	DECLARE rowCount INT;
	DECLARE oldVal INT;
 
	SET count = (SELECT COUNT(*) FROM tabla);
 
	WHILE (rowCount > index) DO
		SET oldVal = (SELECT valor FROM tabla WHERE id = rowCount-1);
		UPDATE tabla SET valor = valor - oldVal WHERE id = rowCount;
		SET rowCount = rowCount -1;
	END WHILE
END //

DELIMITER //
 
DROP PROCEDURE `changeVal`
CREATE PROCEDURE `changeVal`()
 
BEGIN
	DECLARE index INT DEFAULT 2;
	DECLARE count INT;
	DECLARE oldVal INT;
	DECLARE myRow INT;
 
	SET count = (SELECT COUNT(*) FROM tabla);
 
	WHILE (count > index) DO
		SET oldVal = (SELECT valor FROM tabla LIMIT 1, count-2);
		SET myRow = (SELECT id FROM tabla LIMIT 1, count-1);
		UPDATE tabla SET valor = valor - oldVal WHERE id = myRow;
		SET count = count - 1;
	END WHILE
END //