Modifying a CO order

Why the quantity in the first leg of CO order, which is still in the OPEN state, cannot be modified? My requirement is that if a order doesn't get completely filled within a duration (i.e., not in COMPLETE state), then:
  1. if the order is not filled at all (i.e., filled_quantity == 0), then cancel order. (No problem here)
  2. if the order is partially filled (i.e., filled_quantity != 0), then modify the ordered quantity to existing filled_quantity
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