With CommSec at least, reducing your order size doesn't push it to the back. It simply amends the size and you keep your place in the queue.
If you make your order larger, the original volume will remain in the same spot and the additional will be tacked onto the 'back of the queue' . Has always worked that way in my experience and observations.
Can be handy if you are in a big stack.. Hedge your bets by having two spots in the queue - which you can amended/reduce if need be.
