In case you guys think I'm cheating by changing the nullability of the id column, this version is a true replacement, and is just as fast:

SET NOCOUNT ON;

-- Conditional drops

IF OBJECT_ID(N'tempdb..#my_tab', N'U') IS NOT NULL DROP TABLE #my_tab;

IF OBJECT_ID(N'tempdb..#my_tab', N'U') IS NOT NULL DROP TABLE #my_tab2;

IF OBJECT_ID(N'tempdb..#my_tab', N'U') IS NOT NULL DROP TABLE #my_tab3;

-- Sample data

SELECT TOP (100000)

CAST(NULL AS INT) AS ID,

DATEADD(mi,-ABS(CHECKSUM(NEWID())%100000),GETDATE()) AS Any_Date

INTO #My_Tab

FROM master.sys.all_columns T1, master.sys.all_columns T2, master.sys.all_columns T3;

-- Start timing

DECLARE @now DATETIME;

SET @now = GETDATE();

-- Bulk logged identity add (id column is NOT NULL)

SELECT id = IDENTITY(INT, 1000, 1),

any_date

INTO #my_tab2

FROM #my_tab WITH (TABLOCK);

-- Intermediate table for nullability change

CREATE TABLE #my_tab3 (id INT NOT NULL, any_date DATETIME NULL);

-- Lose the IDENTITY attribute

ALTER TABLE #my_tab2 SWITCH TO #my_tab3;

-- Metadata change to id column nullability

ALTER TABLE #my_tab3 ALTER COLUMN id INT NULL;

-- Fast clear original table

TRUNCATE TABLE #my_tab;

-- Metadata switch data in

ALTER TABLE #my_tab3 SWITCH TO #my_tab;

-- Show duration

PRINT 'Duration for 100000 values: ' + CONVERT(VARCHAR(20), DATEDIFF(MILLISECOND, @now, GETDATE()));

-- Tidy up

DROP TABLE

#my_tab, #my_tab2, #my_tab3;