C Minor to C Major

The inflection from minor to major with the same tonic is an old topos in music. Before tonal harmony was fully established the so-called tierce de Picardie (probably a mistaken etymology from tierce piqart meaning 'sharp' third) was often found, ending a piece in minor mode with a major harmony. But in the late 18th century the contrast between minor and major was used in some spectacular ways. Mozart cast his 'Dissonant' quartet introduction in a strange mixture of modes that only is clarified when the movement proper begins, at the 1:05 mark:

Here is the opening of Haydn's The Creation where chaos is depicted with empty octaves and then minor mode:

When God says "let there be light", C major bursts forth gloriously--just after the 2 minute mark in the next clip:

Taking this idea to a more intense level Beethoven writes his 5th Symphony in C minor, but casts the finale, as if a triumph against great odds, in C major. The whole symphony is organized in this way, but the last two movements show the transition from C minor to C major which occurs at 8:36 in this clip. Alas, the last movement is cut short, so I will put up another clip just of the finale.

That was the first time trombones were used in a symphonic, as opposed to operatic, context. All these examples have made the same move, from minor to major, in the same key: C. But there is one final example where the idea is distilled down to an essential form:

After that, the idea was pretty much exhausted. At least in its C minor to C major form in an orchestral context. It had been used so powerfully and so definitively that composers no longer wanted to touch it. But in a different kind of context and in a much less dramatic way, the idea of fluctuating between major and minor continued to be used to express the fluctuating fortunes of a romantic relationship. But now in A minor and major, not C: