PostgreSQL => ALTER TABLE without timezone -> with timezone, using select for TZ

Question

I'm trying to convert the column, and preserve the data in the specified timezone. I have a script, but it errors out on the SELECT at the end.

ALTER TABLE schema.table 
ALTER COLUMN column_date TYPE TIMESTAMP WITH TIME ZONE
USING column_date AT TIME ZONE (SELECT value FROM schema.table WHERE id = 'timezone');

ERROR:  cannot use subquery in transform expression

I have time zones stored, but trying to pull that back to apply to the script is posing a challenge. I know I can simply hardcode the timezone as 'EST|CST|PST', etc. but I have multiple databases this needs to be applied to (with multiple alters per DB), hence the required SELECT at the end. Is there a way to accomplish this?

I've poured over a few questions on this subject; while some came close, they don't quite meet the needs (My apologies if this is a duplicate, I've spent a while searching for an answer).

[RESOLVED]: Using 2 separate queries

ALTER TABLE schema.table 
ALTER COLUMN column_date TYPE TIMESTAMP WITH TIME ZONE;

UPDATE schema.table 
SET column_date = column_date AT TIME ZONE 
    (SELECT value FROM schema.table WHERE id = 'timezone');

Answer 1

You can use dynamic SQL:

DO
$$BEGIN
   EXECUTE format('ALTER TABLE ... AT TIME ZONE %L',
                  (SELECT value FROM atable ...));
END;$$;