I was listening to Green Light by Lorde, during which there is a modulation from A major to the mixolydian mode. DOes that count as a 'key change' per se?
Is this all about the use of a bVII chord in the final choruses? Just let it BE the bVII in the key of A. A chromatic chord doesn't necessarily imply a key change.
Think of a simple blues progression - A7, D7, A7, E7, D7, A7. You have no problem with that being in the key of A, despite the Cnat and Gnat notes do you?
In the event of a modulation, the music is still with the same key signature, as modulations are generally a temporary move. With a key change, the term says it all - the key has changed, it's not temporary, there needs to be a sign displaying the new key.
'Borrowing', from parallel key and other modes with the same root - as in A major using A Dorian, A minor, A Mixolydian etc., is going to be a modulation, and there will be the necessary accidentals in place, with the feeling that the piece will eventually return to the original key. Other common modulations are moving to a related key, a fourth or fifth away, and relative major/minor changes.
A key change occurs whenever the key signature changes. This is pretty objective and quite different from a modulation, which can occur with or without a key change (often with accidentals). If you are switching modes but not tonal center, I would not call that a modulation; I would call it "borrowing (from a parallel scale)."
This is another example of terminology trying to put hard and fast lines where none exist. If the "tonal center" changes for just one chord, it's not a modulation; if an entire movement is around a new tonal center, it's modulation. But everything in between exists as well, and there's no convention (nor could there be reasonably) to say exactly when "modulation" occurs.
Changing from C Ionian to D Dorian is changing mode but isn't changing key.
Changing from C Ionian to D Phrygian is changing mode and changing Key.
D Dorian > E Phrygian - Changing mode, not changing Key.
A Aeolian > G Mixolydian - Changing mode, not changing Key.
D Dorian > Db Lydian - Changing mode, changing Key.
F Aeolian > A Mixolydian - Changing mode, changing Key.
Blue in green by Bill evans is...
D Dorian is in Cmaj
C Dorian is in Bbmaj
Bb Ionian is in BbMaj
A Phrygian is in FMaj
D Dorian is in Cmaj
F Aeolian is in Abmaj
A mixolydian is in Dmaj
The best modal music changes Key and mode. Changing mode is not as weird as you think it's basically changing chords within a key. The cool stuff comes when you change mode but to another key.
I've not listened to Green Light by Lorde but would assume it's just changing mode and not the key, which is kinda boring.
Also you could go:
D dorian > C dorian
But this is just a key change in my opinion and not a mode change.