CREATE TABLE Foo (
x INT,
ch CHAR(1),
dt DATETIME);

INSERT INTO Foo VALUES (5, 'a', '20080101');
INSERT INTO Foo VALUES (9, 'a', '20080105');
INSERT INTO Foo VALUES (7, 'a', '20080108');
INSERT INTO Foo VALUES (8, 'b', '20080210');
INSERT INTO Foo VALUES (6, 'b', '20080201');

WITH CTE
AS (SELECT x,
ROW_NUMBER() OVER(
PARTITION BY ch
ORDER BY dt) AS x_new
FROM Foo)
UPDATE CTE
SET x = x_new;