Changing primary key int type to serial

Is there a way to change existing primary key type from int to serial without dropping the table? I already have a lot of data in the table and I don't want to delete it.

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 ;-)