Excuse me for interjecting, but where do you hear a shift from major to minor?

There is a minor chord played in several places, but I'm not sure that there is a complete shift from major to minor. It looks like the basic chords are I, II, and iii, but then it's I, II, III. The other chords are just variations on those, like with an added 9th or 7th or 2nd, and suspended chords of course.