>I suppose it’s possible to change the sound of an entire piece of music just by changing the key signature, but does anyone actually do that?
>How would that work for music that also uses notes outside the scale? These seem more like questions of composition, which I definitely don’t know anything about.
From wikipedia:
Although transpositions are usually written out, musicians are occasionally asked to transpose music "at sight", that is, to read the music in one key while playing in another. Musicians who play transposing instruments sometimes have to do this (for example when encountering an unusual transposition, such as clarinet in C), as well as singers' accompanists, since singers sometimes request a different key than the one printed in the music to better fit their vocal range (although many, but not all, songs are printed in editions for high, medium, and low voice).
There are three basic techniques for teaching sight transposition: interval, clef, and numbers ...
I don't think the author was referring to transposition there. I think that was about leaving the notes on the staff the same but just changing the key signature. This is done sometimes as a novelty, and the best example is changing something from a major key to a minor key or vice versa.
The article has a link to a recording of Für Elise in a major key [1], and there are many similar renditions of other pieces around. You could in principle do this with any of the seven modes, not just major and minor.
As for notes outside the scale, it seems like these key-signature-changing compositions typically keep them the same (like the D# in Für Elise).
>How would that work for music that also uses notes outside the scale? These seem more like questions of composition, which I definitely don’t know anything about.
From wikipedia:
Although transpositions are usually written out, musicians are occasionally asked to transpose music "at sight", that is, to read the music in one key while playing in another. Musicians who play transposing instruments sometimes have to do this (for example when encountering an unusual transposition, such as clarinet in C), as well as singers' accompanists, since singers sometimes request a different key than the one printed in the music to better fit their vocal range (although many, but not all, songs are printed in editions for high, medium, and low voice).
There are three basic techniques for teaching sight transposition: interval, clef, and numbers ...
[*] https://en.wikipedia.org/wiki/Transposition_(music)#Sight_tr...