Hello, yes you can change / increase your bid at any time. You go to manage flight and can increase your bid. The same slider opens up with your current bid and you move the slide to the new amount.
I have just made my first bid for upgrade ona return flight Bangkok / Brisbane. I can not find any way of knowing if you can be outbid or where your current bid sits and from my current research the bid can be accepted up to 4 hours prior to departure, you will receive an email with advice and emailed a new itinerary. At the time you make a bid you have to also hit a payment button which takes a $1 dollar capture from your CC, or method you chose to pay.
