default for column "xxxx" cannot be cast automatically to type boolean in Postgres DB

You have to drop default constraint before changing the type:

ALTER TABLE parts 
    ALTER COLUMN is_dpm_scanned DROP DEFAULT,
    ALTER COLUMN is_dpm_scanned TYPE BOOLEAN USING is_dpm_scanned::BOOLEAN,
    ALTER COLUMN is_dpm_scanned SET DEFAULT FALSE;

See also: Changing a column from string to string array in postgresql.