A script that outputs the raw SQL to convert Django's serial columns to identity columns.

serial-to-identity.sql

WITH tab AS (
    SELECT
        a.attrelid::regclass::text AS t,
        a.attname AS c,
        pg_get_serial_sequence(a.attrelid::regclass::text, a.attname) AS s,
        nextval(pg_get_serial_sequence(a.attrelid::regclass::text, a.attname)) AS v
    FROM pg_attribute a
        JOIN pg_class c ON c.oid = a.attrelid
    WHERE
        a.attrelid::regclass::text LIKE '%'
        AND c.relkind IN ('r', 'p')  /* regular and partitioned tables */
        AND a.attnum > 0
        AND NOT a.attisdropped
        AND a.atttypid = ANY ('{int,int8,int2}'::regtype[])
        AND EXISTS (
            SELECT FROM pg_attrdef ad
            WHERE
                ad.adrelid = a.attrelid
                AND ad.adnum = a.attnum
                AND (
                    pg_get_expr(ad.adbin, ad.adrelid)
                    =
                    'nextval('''
                    || (
                        pg_get_serial_sequence(a.attrelid::regclass::text, a.attname)
                    )::regclass
                    || '''::regclass)'
                )
        )
    ORDER BY a.attnum
)
SELECT
    'BEGIN; '
    || 'LOCK TABLE ' || quote_ident(t) || ' IN ACCESS EXCLUSIVE MODE; '
    || 'ALTER TABLE ' || quote_ident(t) || ' ALTER COLUMN ' || quote_ident(c) || ' DROP DEFAULT; '
    || 'DROP SEQUENCE ' || s || '; '
    || 'ALTER TABLE ' || quote_ident(t) || ' ALTER ' || quote_ident(c) || ' ADD GENERATED BY DEFAULT AS IDENTITY (RESTART ' || v || '); '
    || 'COMMIT;'
FROM tab;

Unless I'm mistaken, this appears to get the nextval of the old sequence at the the SQL is generated, rather than at the time the generated SQL itself it run. That means that if any rows are inserted in-between the time the SQL is generated and when it's run, the new sequence value will be incorrect. Shouldn't the call to nextval be part of the generated SQL (and inside the exclusive table lock)? That'd also mean re-ordering commands so that the DROP SEQUENCE statement is after the last ALTER TABLE.

Update: Here's an updated version of this script that fixes the issue w/ nextval timing.

WITH tab AS (
    SELECT
        a.attrelid::regclass::text AS table_name,
        a.attname AS pk_col,
        pg_get_serial_sequence(a.attrelid::regclass::text, a.attname) AS seq_name
    FROM pg_attribute a
    JOIN pg_class c ON c.oid = a.attrelid
    WHERE
        a.attrelid::regclass::text LIKE '%'
        AND c.relkind IN ('r', 'p')  /* regular and partitioned tables */
        AND a.attnum > 0
        AND NOT a.attisdropped
        AND a.atttypid = ANY ('{int,int8,int2}'::regtype[])
        AND EXISTS (
            SELECT FROM pg_attrdef ad
            WHERE
                ad.adrelid = a.attrelid
                AND ad.adnum = a.attnum
                AND (
                    pg_get_expr(ad.adbin, ad.adrelid)
                    =
                    'nextval('''
                    || (
                        pg_get_serial_sequence(a.attrelid::regclass::text, a.attname)
                    )::regclass
                    || '''::regclass)'
                )
        )
    ORDER BY a.attnum
)
SELECT
    'BEGIN; '
    || 'DO $$ DECLARE nextSeqVal bigint; '
    || 'BEGIN '
    || 'LOCK TABLE ' || quote_ident(table_name) || ' IN ACCESS EXCLUSIVE MODE; '
    || 'ALTER TABLE ' || quote_ident(table_name) || ' ALTER COLUMN ' || quote_ident(pk_col) || ' DROP DEFAULT; '
    || 'nextSeqVal := nextval(''' || seq_name || '''); '
    || 'DROP SEQUENCE ' || seq_name || '; '
    || 'EXECUTE format(''ALTER TABLE ' || quote_ident(table_name) || ' ALTER ' || quote_ident(pk_col) || ' ADD GENERATED BY DEFAULT AS IDENTITY (RESTART %s);'', nextSeqVal); '
    || 'END $$; '
    || 'COMMIT;'
FROM tab;

As well, here's an example bash script of how you can generate and run the migration code on a batch of DBs:

#!/usr/bin/env bash

set -euxo pipefail

DB_URL="postgres://foo:[email protected]:5432"

for DB in foo bar baz
do
    psql \
        -d "${DB_URL}/${DB}" \
        -f serial-to-identity.sql \
        | grep 'BEGIN;' \
        | awk '{$1=$1};1' \
            > "migrate-${DB}.sql";
    psql -d "${DB_URL}/${DB}" \
        -f "migrate-${DB}.sql";
    echo "Finished ${DB}";
done

This should execute directly

DO $$
DECLARE
    rec record;
    table_ident text;
    seq_ident text;
    max_val bigint;
BEGIN
    FOR rec IN
        SELECT
            a.attrelid::regclass::text AS t,
            a.attname AS c,
            pg_get_serial_sequence(a.attrelid::regclass::text, a.attname) AS s
        FROM pg_attribute a
        JOIN pg_class c ON c.oid = a.attrelid
        WHERE
            a.attrelid::regclass::text LIKE '%'
            AND c.relkind IN ('r', 'p')  -- regular and partitioned tables
            AND a.attnum > 0
            AND NOT a.attisdropped
            AND a.atttypid = ANY ('{int,int8,int2}'::regtype[])
            AND EXISTS (
                SELECT 1
                FROM pg_attrdef ad
                WHERE
                    ad.adrelid = a.attrelid
                    AND ad.adnum = a.attnum
                    AND (
                        pg_get_expr(ad.adbin, ad.adrelid)
                        =
                        'nextval('''
                        || (pg_get_serial_sequence(a.attrelid::regclass::text, a.attname))::regclass
                        || '''::regclass)'
                    )
            )
        ORDER BY a.attnum
    LOOP
        -- Handle schema-qualified table names
        table_ident := CASE 
                         WHEN position('.' in rec.t) > 0 
                           THEN quote_ident(split_part(rec.t, '.', 1)) || '.' || quote_ident(split_part(rec.t, '.', 2))
                         ELSE quote_ident(rec.t)
                       END;
        -- Handle schema-qualified sequence names
        seq_ident := CASE 
                       WHEN position('.' in rec.s) > 0 
                         THEN quote_ident(split_part(rec.s, '.', 1)) || '.' || quote_ident(split_part(rec.s, '.', 2))
                       ELSE quote_ident(rec.s)
                     END;
        
        -- Lock the table to prevent concurrent modifications
        EXECUTE 'LOCK TABLE ' || table_ident || ' IN ACCESS EXCLUSIVE MODE';
        
        -- Get the max value of the column to set the new identity restart value
        EXECUTE 'SELECT COALESCE(MAX(' || quote_ident(rec.c) || '), 0) + 1 FROM ' || table_ident INTO max_val;
        
        -- Drop the default and sequence
        EXECUTE 'ALTER TABLE ' || table_ident || ' ALTER COLUMN ' || quote_ident(rec.c) || ' DROP DEFAULT';
        EXECUTE 'DROP SEQUENCE ' || seq_ident;
        
        -- Convert the column to GENERATED ALWAYS AS IDENTITY
        EXECUTE 'ALTER TABLE ' || table_ident || ' ALTER COLUMN ' || quote_ident(rec.c)
                || ' ADD GENERATED ALWAYS AS IDENTITY (RESTART WITH ' || max_val || ')';
    END LOOP;
END $$;