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Easiest? Add a new line to your score for that instrument, and copy, paste. That takes care of any transposition, etc. if needed. You can mute or delete the previous instrument - it is saved in your auto backups anyway. And - go look at DORICO. My adult son is also in the music business (4 of us in this family are) and after ages with Sibelius, he had changed over and RAVES. Had his Master's degree in digital music and composition.