One should really talk about 'one tone lower' or 'one semitone lower'.
In this respect 'one note lower' is meaningless, as organ scales are
not chromatic.

If E becomes D then that is a drop of one tone, so F cannot become E,
since this is only a semitone.  F should become D-sharp and F-sharp
becomes E.

Alternatively, you could drop each note by a semitone.

Regards from Great Britain,
Nicholas Simons