DECLARE @a TABLE ( OrderId INT, Col CHAR(1) )
INSERT @a ( OrderId, Col )
SELECT 1, 'A' UNION
SELECT 2, 'B' UNION
SELECT 3, 'C' UNION
SELECT 4, 'D' UNION
SELECT 5, 'E' UNION
SELECT 6, 'F'

DECLARE @Order INT, @Col CHAR(1)

SELECT @Order = 4, @Col = 'A'

UPDATE  @a
SET     OrderId = rn
FROM    @a a
        JOIN ( SELECT   ROW_NUMBER() OVER ( ORDER BY OrderId, NewInd ) AS rn,
                        Col
               FROM     ( SELECT    OrderId,
                                    NULL AS NewInd,
                                    Col
                          FROM      @a
                          WHERE     Col <> @Col
                          UNION
                          SELECT    @Order,
                                    1,
                                    @Col
                        ) a
             ) b ON a.Col = b.Col

SELECT * FROM @a
ORDER BY OrderId