You have to drop the 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 for a detailed explanation.
- How to cast varchar to boolean.