UpdateByCopyRename.sql

-- Create a new version of the table
CREATE TABLE dbo.Example2
(
    SomeID      integer NOT NULL,
    StartDate   date NOT NULL,
    EndDate     date NOT NULL
);
-- Add the appropriate clustered index
CREATE UNIQUE CLUSTERED INDEX CX_Example_SomeID_StartDate 
ON dbo.Example2 (SomeID, StartDate)
WITH (MAXDOP = 1);
-- Copy the data into the new table, similar to the Manual Halloween Protection approach
INSERT INTO dbo.Example2 WITH (TABLOCK) (SomeID, StartDate, EndDate)
SELECT 
    E.SomeID,
    E.StartDate,
    ISNULL(DATEADD(DAY, -1, 
        LEAD(E.StartDate) OVER (
            PARTITION BY E.SomeID
            ORDER BY E.StartDate)), CONVERT(date, '99991231', 112)) AS EndDate
FROM dbo.Example AS E WITH (TABLOCK)
OPTION (MAXDOP 1);
-- Drop (or, alternatively rename out of the way, the old table)
DROP TABLE dbo.Example;
-- Rename the new table to take the place of the old table
EXEC sp_rename 'dbo.Example2', 'Example';