Answers for "how to alter column to be constraint in postgres"

alter column in table postgres

ALTER TABLE table_name
  ALTER COLUMN column_name TYPE column_definition;

alter constraint postgres

/* Drop your constraint */
alter table yourtable drop constraint constraint_name;

/* Create new constraint */
alter table yourtable add constraint "constraint_name" FOREIGN KEY (some_id) REFERENCES yourtable(some_id) on delete cascade;