persian.ly and transposition

[Kirtap Akud]

I have a persian music score in Shur D.
I want the written score to stay as it is but I want the midi file to sound 1/2 tone lower, without having to change all the notes of the score.


In the persian.ly there's an indication about transposition: "If  you want to use  the \transpose command  you'll have to
manually adjust the key signature of the scale."
I'm not sure if that  and I don't know how to "manually adjust the key signature of the scale."

What must I do to keep the written score as it is and have the midi file sound 1/2 tone lower?
I'm not familiar with the \transpose command but I've tried it without success, is there a special trick for the persian.ly?

thanks,
Patrick.