then try this
DECLARE @offset INT
select @offset=isnull(max(RowID),0) from Customers
MERGE Customers AS C
USING (SELECT ROW_NUMBER() OVER ORDER BY ID) AS Seq,*
FROM NewCustomers) AS NC
ON C.LastName = NC. LastName
WHEN MATCHED THEN
UPDATE SET C.FirstName = NC.FirstName
WHEN NOT MATCHED THEN
INSERT (LastName, FirstName,RowID) VALUES (NC.LastName,NC.FirstName,NC.Seq +@offset);
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SQL Server MVP
http://visakhm.blogspot.com/