make_own_squence.sql

-- Converting an int to a serial more or less only means adding a sequence default to the value, so to make it a serial;

-- Pick a starting value for the serial, greater than any existing value in the table
SELECT MAX(id)+1 FROM mytable

-- Create a sequence for the serial (tablename_columnname_seq is a good name)
CREATE SEQUENCE test_id_seq MINVALUE 3 (assuming you want to start at 3)

-- Alter the default of the column to use the sequence
ALTER TABLE test ALTER id SET DEFAULT nextval('test_id_seq')

-- Alter the sequence to be owned by the table/column;
ALTER SEQUENCE test_id_seq OWNED BY test.id