This post features Fernando Sor’s well-known Study in C, Opus 60, No. 1, recomposed in the key of C minor. (Compare both versions below.) Converting a melody to a parallel major or minor, involves a bit more than just changing the key signature. An important aspect to consider is the harmony that the melody implies. For example, scale degree 7 (the leading tone) should be raised in a minor key to reflect any implied dominant chords. (All but one of the B’s in the recomposed version of the Sor piece are still natural). Note that the first two measures of both versions are identical such that the key of the piece is not truly aurally established until m. 3. If you’re familiar with the C major version, the C minor version requires a bit of an adjustment upon a first listening!
One additional subtle change to the minor version of the piece involves enhancing the harmony in m. 7. The change from A to A-flat on beat 1 automatically creates an implied augmented 6th harmony for that measure (A-flat/F-sharp), a chord that is well-suited for a minor key. Subsequently, replacing the original pitch D with an E-flat on beat 3 signals more intensely the arrival of the dominant in m. 8.