PERFORM * FROM ((TABLE old_table EXCEPT TABLE new_table) UNION ALL (TABLE new_table EXCEPT TABLE old_table)) AS differences LIMIT 1; IF FOUND THEN ... changed ... END IF;
Maybe converting new and old records to json and text
PERFORM * FROM (select ID, row_to_json(O.*)::text Old_Values, row_to_json(N.*)::text New_Values from old_table o full outer join new_table N using(ID) where Old_Values is distinct from New_Values) as differences LIMIT 1;