156. Subject to such restrictions as may be contained in any warrant issued under section 152 or section 153, a confirming authority may, when confirming the sentence of a court-martial, mitigate or remit the punishment thereby awarded, or commute that punishment for any punishment or punishments lower in the scale laid down in section 73: Provided that a sentence of imprisonment shall not be commuted to a sentence of detention for a term exceeding the term of imprisonment awarded by the court.