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
A very simple SQLfiddle demo.
And as always, make a habit of running a full backup before running altering SQL queries from random people on the Internet 😉