Arrangement (music)
Sometimes composers have made changes to the orchestration of works for orchestra written by others.  They may have done this because instruments had changed since the music was written, or simply because they thought they could do it better.  For example, Mozart made an arrangement of Handels  Messiah , adding instruments such as clarinets which Handel would not have known.  Rimsky-Korsakov made arrangements of the operas by Musorgsky thinking that he was improving them.