Rename a column if not already renamed.?

I have a function used by my patch files which attempts to determine if the old column name exists, if so it  proceeds
withthe rename.  


CREATE OR REPLACE FUNCTION sys.rename_column(schema_table_ regclass, old_name_ text, new_name text)
 RETURNS boolean
 LANGUAGE plpgsql
AS $function$
BEGIN

IF EXISTS(SELECT 1 FROM pg_attribute
          WHERE attrelid = schema_table_
          AND attname = old_name_
          AND NOT attisdropped) THEN
    EXECUTE format ('ALTER TABLE %s RENAME %s TO %s', schema_table_, old_name_, new_name);
    RETURN TRUE;
ELSE
   RETURN FALSE;
END IF;

END;
$function$;


This seems to function correctly except.

If I am upgrading a site from Postgres 9.6 to version 11.3 and the column has already been renamed in the 9.6 branch
theabove existence test fails 
and an exception is generated.

It all seems to work correctly if I repeat this same patch in the 11.3 branch.

The function definition is the same for both branches.

I suspect I am overlooking some fundamental issue here.
Anyone with a thought.


Thanks


Dave Day