Join Date: Jul 2001
Location: Berlin, Germany
The C -> Am change would not change the notes you play (the most obvious scale choices are C-major and A-minor respectively, same notes, different root). If were to be playing over this change, you could either use a different, less obvious, scale over one of the chords, or to accentuate the change you could try to accentuate the two different triads.
Back to the question, I don't tend to think much in modes, I would usually play the C-major scale over the C-major chord, D-major over D-major chord, G-major over G, A-minor for Am, etc...
So, yes in a way the notes you are using are relative to the root, (WWHWWWH formula for major scales, and WHWWHWW for minor scales), you are playing all notes from the root to the octave using these formulas. Of course, you could think of playing over an Am chord, as using a major scale, the C (the same way you could think of playng over an Em chord, as playing a G-major scale), but it's better to think in major scales as well IMO.
I don't know if this is making sense anymore, but I would say that if your progression is
all being major chords, the obvious thing to do would be to play C-ionian, A-ionian, D-ionian, G-ionian.
Than the scale choices are, C-ionian, A-aeolian (same notes as C-ionian), D-aeolian (same notes as F-ionian), G-ionian.
And so on...
Then when you come to varioius 7, maj7, m7, m7b5, 6 etc chords, the system works in a similar way, but your choice of scales changes, in some cases.
I would advice you to try to "see" scale patterns in chord shapes (either thinking "by dots", or thinking of intervals, whatever is easier for you), this would make it simpler for you to solo over more exotic chords without neccesarily having to memorise what the "correct" scale choice is (there is never one correct choice anyway).
Ok, enough of this before I confuse you and myself even more.