Here's a concrete example of the question, which may help some readers:
If I'm playing a blues in F, I'll play the F blues scale (F-A♭-B♭-B♮-C-E♭) over the I7 chord. Should I continue playing the F blues scale over the IV7 chord? Or should I switch to B♭7 since jazz thinks in terms of chords (not in terms of a single key center)?
tldr; it depends on the genre, the complexity of the music, and personal preferences. But many blues/jazz students probably have this exact question, and it's rarely explained.
Consider 3 different examples:
- Example A = Bag's Groove by Milt Jackson
- Example B = Blues for Alice by Charlie Parker
- Example C = 26-2 by John Coltrane
I chose these examples because get increasingly complex, harmonically. Yet, they're all a 12-bar blues (or based on a 12-bar blues form).
Your question is mostly about Example A (low harmonic complexity) and B (medium harmonic complexity). But some of your statements are most applicable to Example B (medium harmonic complexity) or C (high harmonic complexity). We'll go through your statements one-by-one.
- It's in jazz to use the approach of not thinking of a song being in a certain key when it comes to melody and soloing rather that each
chord change uses the scale of the chord.
For Example C, this is 100% true. In Example C, the harmony is so complex that it doesn't help to think of a single key center. But this statement isn't 100% true for Examples A and B. In fact, many people will play the F blues scale (F-A♭-B♭-B♮-C-E♭) over all of Example A. Even as the chords change, they continue to use the F blues scale. For an up-tempo tune where you improvise over 10 iterations through the form, that can get boring. So to mix it up, you can also "play the changes" and improvise using scales that are more specifically matched to the particular chords.
- These chord changes can also be seen as changing modes. So the melody would play the keys of the same keys but changing root note
(and in fact key since its a mode)
This is a little off-track. Using modes definitely does not mean that the song changes key or tonal center. Similarly, having many different chords in a song does not mean the song changes key or tonal centers. Example B is a good illustration of this: some of the chords don't fit in the key of F; but the entire song is still in F as a whole. Measures 6-8 contain the chords | B♭m E♭7 | Am D7 | A♭m7 D♭7 |. However, these chords wouldn't be described as a "key change," because the song as a whole isn't moving to a new tonic / tonal center. When I see m. 6-8 of Example B, I think "the song is in F, but we have some descending ii-V's that will require me to play the changes."
This gets to the heart of modes, what they are, and why they're helpful. Modes help us try new sounds and expand our harmonic/melodic vocabulary. They categorize the different options that are available to us when we want to "play over the changes." They provide a formal way of thinking about the different choices of scales that we could use when we see a given chord symbol. They also give us a way to think about how much harmonic complexity our improvisation introduces. Imagine we're improvising over Example A, and we want our improvisation to get gradually more complex. We could think about modes: the first time through the form, we solo with an F blues scale the whole way. The second time through the form, we use Mixolydian modes. The third time, we use a mix of Lydian Dominant and Altered Dominant scales. This might sound too formulaic, but with time, we can achieve this sort of progression smoothly and with good cohesion, and it can be a helpful high-level cognitive tool for thinking about constructing improvisations with interesting harmonic progression.
- It's also common to use pentatonic scales with added notes that sound extra bluesy or jazzy.
This is very true. The blues scale is a good example of this. For instance, the F blues scale is F-A♭-B♭-B♮-C.
- Finally a question. These special notes would they change with the key of the chord that is playing? So if such a note is the 4b (or 5#?) and the current chord is C then the special note would be F#. If the next chord is G then C# would have the same role?
This depends on the complexity. In Example C, the notes would absolutely change with the chords. In Example A, they don't have to. For instance, with a simpler blues song, you can play the F blues scale (F-A♭-B♭-B♮-C-E♭) over both the F7 chord and the B♭7 chord. But it depends on personal style, and on the genre. For example, in blues music, you will often hear people play an F blues scale over the F7 chord, and then they'll switch to a B♭ blues scale over the B♭7 chord. In contrast, you almost never hear this in bebop jazz. So it depends. But one thing we can say is: there are songs in jazz (like Example C) where the harmonic complexity is so high that you could never simply play an F blues scale over the entire form; instead, you have to think about playing the changes. For example, maybe you're playing the song 26-2, and your idea is to solo using major bebop scales. You can play F maj bebop (F-G-A-B♭-C-D♭-D♮-E-F) in m. 1, and you'll need to switch to D♭maj bebop in m. 2.