My understanding is that if your upgrade bid is accepted, they send you a notification including the pertinent information and amount to pay. I think you have the ability to decline. And if you don't pay within the time allotted, then the upgrade offer expires and you remain in your currently book cabin...
Per "Bid for a Stateroom Upgrade FAQ"
"What happens when my bid is accepted?
You’ll receive a bid acceptance email letting you know that your upgrade will be processed, and you’ll receive a new booking confirmation within two business days. This confirmation will provide your new stateroom location and your balance is due within 48 hours."
Does someone have any information on this or are able to confirm/clarify this?