If you increase the quantity of shares there will be two orders in the queue:
The first will be your original order and you will retain your existing position in the queue.
The second will be your new (additional) quantity of shares, which will be placed at the end of the queue.
Your broker will/should treat it as single order for calculating commission and GST.
Have been through the above and also queried how amended share parcel with increased quantities are handled.
Not sure what happens on a decrease in the number of shares.
You could query your broker (online or full-service) for a definitive answer.
Can think of two logical approaches, but it is better to obtain the correct answer.