Bigserial does not existAlthough you can create tables with columns of Bigserial you can't alter columns to said type. I got around it like so...
CREATE SEQUENCE my_column_seq;
ALTER TABLE my_table ALTER COLUMN my_column_id TYPE bigint, ALTER COLUMN my_column_id SET default nextval('my_column_id_seq'::regclass)
Either select the current max sequence value or make sure the sequence is set to one more than the current max my_column_id, e.g. if it's 500, set to 501.
SELECT setval('my_column_id_seq', (SELECT max(my_column_id) FROM my_table));
ALTER sequence my_column_id_seq RESTART WITH 501 INCREMENT BY 1