--using SQLCMD mode
:ON ERROR EXIT 
SET XACT_ABORT ON;

BEGIN TRANSACTION;

CREATE TABLE #tmp (
	ID int,
	A bit NOT NULL, 
	B bit NOT NULL,
	C bit NOT NULL
);

INSERT INTO #tmp(ID, A, B, C)
SELECT ID, A, B, C
FROM dbo.tbl;

ALTER TABLE dbo.tbl
DROP CONSTRAINT DF_tbl_A, 
                DF_tbl_B,
                DF_tbl_C;

ALTER TABLE dbo.tbl
ALTER COLUMN A datetime NULL;

GO

ALTER TABLE dbo.tbl
ALTER COLUMN B datetime NULL;

GO

ALTER TABLE dbo.tbl
ALTER COLUMN C datetime NULL;

GO

UPDATE t SET
  t.A = CASE WHEN x.A = 1 THEN GETDATE() ELSE NULL END,
  t.B = CASE WHEN x.B = 1 THEN GETDATE() ELSE NULL END,
  t.C = CASE WHEN x.C = 1 THEN GETDATE() ELSE NULL END
FROM dbo.tbl AS t
INNER JOIN #tmp AS x
  ON t.ID = x.ID;

DROP TABLE #tmp;

ROLLBACK;

CREATE TABLE dbo.tbl (
	ID int,
	A bit NOT NULL DEFAULT (0), 
	B bit NOT NULL  DEFAULT (0),
	C bit NOT NULL  DEFAULT (0)
);

INSERT INTO dbo.tbl 
SELECT 1, 0,0,0 UNION ALL
SELECT 2, 1,0,1 UNION ALL
SELECT 3, 0,1,0 

CREATE TABLE dbo.tbl (ID INT, A BIT, B BIT, C BIT);
GO
ALTER TABLE dbo.tbl ADD CONSTRAINT DF_tbl_A DEFAULT(0) FOR A;
ALTER TABLE dbo.tbl ADD CONSTRAINT DF_tbl_B DEFAULT(0) FOR B;
ALTER TABLE dbo.tbl ADD CONSTRAINT DF_tbl_C DEFAULT(0) FOR C;

GO
INSERT  INTO dbo.tbl
        (ID, A, B, C)
VALUES  (1, 1, 1, 1),
        (2, 0, 0, 0);
GO
SET XACT_ABORT ON;

BEGIN TRANSACTION;

CREATE TABLE #tmp
    (ID INT,
     A BIT NOT NULL,
     B BIT NOT NULL,
     C BIT NOT NULL);

INSERT  INTO #tmp
        (ID, A, B, C)
        SELECT  ID,
                A,
                B,
                C
        FROM    dbo.tbl;

ALTER TABLE dbo.tbl
DROP CONSTRAINT DF_tbl_A, 
DF_tbl_B,
DF_tbl_C;

ALTER TABLE dbo.tbl
ALTER COLUMN A DATETIME NULL;

GO

ALTER TABLE dbo.tbl
ALTER COLUMN B DATETIME NULL;

GO

ALTER TABLE dbo.tbl
ALTER COLUMN C DATETIME NULL;

GO

UPDATE  t
SET     t.A = CASE WHEN x.A = 1 THEN GETDATE()
                   ELSE NULL
              END,
        t.B = CASE WHEN x.B = 1 THEN GETDATE()
                   ELSE NULL
              END,
        t.C = CASE WHEN x.C = 1 THEN GETDATE()
                   ELSE NULL
              END
FROM    dbo.tbl AS t
        INNER JOIN #tmp AS x
            ON t.ID = x.ID;

DROP TABLE #tmp;

COMMIT;
GO
DROP TABLE dbo.tbl;

We walk in the dark places no others will enter
We stand on the bridge and no one may pass