Complex sentence: As John Williams chose not to return to score the film due to obligations with Lucasfilm's "The Empire Strikes Back" and "Raiders of the Lost Ark", instead Ken Thorne was commissioned to write the music upon Williams' recommendation.
Split into simpler sentences: As John Williams chose not to return to score the film due to obligations with Lucasfilm's "The Empire Strikes Back" and "Raiders of the Lost Ark". Instead, Ken Thorne was commissioned to write the music upon Williams' recommendation.