This query might work for you, but double check all result statements first.
SELECT 'ALTER TABLE ' || quote_ident(n.nspname) || '.' || quote_ident(c.relname)
|| ' ALTER COLUMN ' || quote_ident(a.attname) || ' TYPE varchar;'
FROM pg_class c
JOIN pg_namespace n ON n.oid = c.relnamespace
JOIN pg_attribute a ON a.attrelid = c.oid
JOIN pg_type t ON t.oid = a.atttypid
WHERE t.typname = 'char'
AND n.nspname <> 'pg_catalog';