If you amend the quantity down, then you retain the place in the queue and the quantity adjusted down.
If you amend the quantity up, then your original quantity retains the position, and the additional quantity that you've added goes to the bottom of the queue, so that others who have been waiting won't lose their position in the queue.
Depends on the broker too, on whether they are actually amending or cancelling and placing a new order for you behind the screens.
Hope that makes sense. If you have any questions like these, I suggest asking the wonderful DT community in the after market or weekend thread, where more people have time to respond. Plus, keeps the DT thread clear too.
