Try this script which modifies a PK column to auto increment.
DROP TABLE TEST
GO
create table test
(
code varchar(100),
DESCR varchar(1000)
CONSTRAINT P_test_code PRIMARY KEY CLUSTERED(code)
)
go
insert into test(code, DESCR)
select 'A', 'AAAA'
UNION ALL
select 'B', 'BBBBB'
go
select *
from test
go
ALTER TABLE test DROP CONSTRAINT P_test_code
GO
ALTER TABLE test ADD CODE2 INT IDENTITY
GO
ALTER TABLE test
ADD PRIMARY KEY (CODE2)
go
select *
from test