Postgres 13

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create table my_table (id serial primary key, my_column boolean);

create type my_type as enum ('enabled', 'disabled', 'inherited');

CREATE TABLE

CREATE TYPE

insert into my_table (my_column) values (true), (false), (null)

INSERT 0 3

select * from my_table

id
my_column

1
t

2
f

3
null

SELECT 3

alter table my_table
	alter column my_column set data type my_type using (
		case my_column 
			when true then 'enabled'::my_type
			when false then 'disabled'::my_type
			else 'inherited'::my_type
		end
    )
;

ALTER TABLE

select * from my_table

id
my_column

1
enabled

2
disabled

3
inherited

SELECT 3

id	my_column
1	t
2	f
3	null