Major to Minor

For the third note of the scale, this might be easy, however it is not so easy when determining what to do with the 6th or 7th scale degrees. Many times the 7th would be the same as it is in major, but many times it would be flatted, and occasionally even the 6th would be the same as it is in major.

As this depends on the piece specifically, I don't know if this is something that can really be added. Except possibly if there was an option to A. transpose all 3rds and 6ths (I mean scale degrees, not intervals) down a half step, B. transpose all 3rds, 6ths, and 7ths down a half step, C. leave everything as is. That would probably cover the most desired options.

A feature like this would only work for simple melodies, or harmonics that don't modulate and only use tonic, subdominant and dominant functions. Any music a bit more complex than that can't be easily transformed from major to minor or the other way.
A (quite dirty) hack could save you some work, though:

assuming you are in C major mode:
- select a section of music
- transpose it a minor third down (to A major)
- change key signature to A minor by moving the signature from the palette to the first bar of your section
- click on the "natural" sign in the icon toolbar, your music changed from A major to A minor now
- transpose your section a minor third up again
- change key signature to C minor (three flats)

You have still to check the cases where you would want A natural and B natural rather than A flat and B flat.

This only works, if your piece of music does not use any accidentals (within the piece).
For any other major mode (existing key signature), transpose to A, same procedure, than back to original mode, and change to according key signature

good luck :-)

I can see the novelty value in it but it's a bit like asking a graphics program to change a picture from daytime to night-time not by just changing the darkness and hues but by also switching on the lights in the background and adding car tail-lights. Cool, yes, but probably not the highest priorty for programmers.

You can speed up the process by changing to the corresponding minor or Major key (C Maj = A min etc.) and then transposing up or down by 3 semitones but you still need to listen to it and make some manual adjustments.

The attached file started off as a mistake when I got the Key Signature wrong but I then just changed a few notes to make a minor version of the original (with a bit of artistic license, of course).

I know it's an old thread but I just wanted to comment my solution. My original score was in G Major (it's the Starlight Waltz by the way) and I wanted it in G minor. My solution takes the same first step as the one proposed by @underquark although I hadn't read his comment before trying it myself.

-What I did was first transpose (from G Major) to B♭ Major by using: Tools> Transpose> Transpose by tonality> B♭ Major / G minor.
-I then selected all notes by right-clicking on one note and using Note> Select> All similar elements. I then pressed the downward key on my computer keyboard four times and then once the upward key, until my root note was a natural G; I did that so that all the C flat notes turned into a B natural.
-To get rid of all the B notes that had accidental naturals (it was like this throughout the score) I selected one natural symbol and again used Select> All similar elements and then simply pressed delete to get rid of them. I had to do this twice since there were some bars with more than one B in them, and doing it once just shifted the natural symbols to those second and third B notes.
-I then wanted all my F notes to be sharp so I selected an F note, right-clicked on it and used Note> Select> More> Same note name: F. With all of them selected I pressed the upward key once and they all turned into an F sharp.

There were still some minor adjustments to it, but the whole thing took like 3 minutes so I think that is not too much.