Or do you mean that you’d do it so that the violin plays in E and you would play it in flats? So that the first measure, which is basically E major chords, would be played f-flat, C-flat, A-flat, etc? I guess you could do that, but the piece is so chromatic I’m not at all sure that overall it would be that much of a tonal improvement on a well-regulated harp.

EDIT To clarify, I mean that you’re going to be pedaling all over the darned place either way. I think mostly you’d just be shifting where the pedals are up vs down, not getting all that many more notes in flats, although it’s been years since I last played it, so I’m just going from what I remember.

So is your question simply to play the harp part in Fb major, leaving the other players unchanged, without any transposition; I don’t think that was obvious at first, or was it into Cb major ?

Whilst I don’t know Beau Soir at all, from what I’ve read above I would say that the suggestion simply to drop it one semitone into Eb is probably the best – recalling that Debussy’s 1st Arabesque for piano is in E major and the harp transcription by Renie or Salzedo is in Eb – (will check my facts) – and this way it is possible to play straight from the original sheet music, knowing that accidentals # on the original is a natural on the harp, similarly a natural accidental becomes a flat – so the players could do the same on their parts and change the key signature.
http://imslp.org/wiki/Beau_soir_(Debussy,_Claude)

http://imslp.org/wiki/2_Arabesques_(Debussy,_Claude)

Will the brightness of the key change ? The harp is supposedly more melancholy in flats, I wonder if anybody notices, or the mood drops.

Just a note: Fb major is 8 flats, not 7, so you’d have to contend with playing all of your A flats and B double flats (A naturals) on the same strings.