You can't turn an existing IDENTITY off and you can't make an existing column IDENTITY. However you can add an IDENTITY column to an existing table.
Any indexes should be scripted, dropped and recreated after the change.
-- Setup test table
CREATE TABLE TestIdentity
(ID INT IDENTITY(1, 1),
Value INT);
GO
-- Setup test data
INSERT INTO TestIdentity (Value)
VALUES (1),(2),(3),(4),(5);
GO
-- Add new column
ALTER TABLE TestIdentity
ADD ID2 INT NULL;
GO
-- Copy data
UPDATE TestIdentity
SET ID2 = ID;
GO
-- Remove IDENTITY
ALTER TABLE TestIdentity
DROP COLUMN ID;
GO
-- Rename new column
EXEC sp_rename 'TestIdentity.ID2', 'ID', 'COLUMN';
GO
INSERT INTO TestIdentity (Value)
VALUES (11),(12),(13),(14),(15);
GO
SELECT * FROM TestIdentity;
GO
DROP TABLE TestIdentity;
GO
