作曲 and 編曲: composing, beginning, and arranging an existing song
Compare 作曲 for a film theme and the start of work from a melody with 編曲 for a folk song arranged for a choir or a changed impression. Keep beginnings, completed actions, existing music, and arrangement effects distinct.
作曲: composing a theme or beginning work
作曲 (さっきょく) concerns composing music. 「映画の主題曲を若い音楽家が作曲した。」 means “A young musician composed the movie's theme song.” A young musician composed the film's theme song. That composing role is stated, but acting in or directing the film is not.
「散歩中に浮かんだ旋律をもとに作曲を始めた。」 means “I began composing based on a melody that came to me during a walk.” A melody that came to mind during a walk provided the basis for beginning to compose. 始めた identifies the start of the work, without adding completion of the whole piece.
編曲: changing an existing song for a purpose
編曲 (へんきょく) concerns arranging existing music. 「古い民謡を合唱向けに編曲した。」 means “I arranged an old folk song for a choir.” The object is an old folk song, and the arrangement is for a choir. Working on that existing song should not be retold as composing the folk song from scratch.
「同じ曲でも編曲によって印象が大きく変わる。」 means “Even the same song can feel very different depending on its arrangement.” An arrangement can change the impression of the same song considerably. A different impression does not automatically mean an improvement, nor does the sentence prescribe a particular genre for the change.
Check the object and how far the work proceeded
The 作曲 examples distinguish composing a film theme from beginning work based on a melody. They do not establish a single sequence carried out by the same musician. The theme was composed in one sentence, while the other states a beginning without asserting completion.
For 編曲, connect the existing folk song with the choral purpose, and read the separate statement about an arrangement changing a song's impression. Compare the objects and activities without assuming that every arrangement is better or that there was no original song before the arrangement.
Does 「古い民謡を合唱向けに編曲した。」 say that the folk song was newly composed from scratch?
Show the answer and why
No. It says that an existing old folk song was arranged for a choir. Distinguish 作曲 for composing from 編曲 for arranging existing music. The separate example that begins composing also does not state that the whole piece was finished.