For what it's worth, I actually just bid on an upgrade using this system today.
I booked an ocean view 7-night cabin to Alaska three days ago, having to pay it in full since the sailing was only 3 weeks away. We saw a great deal and went for it. Yesterday, I saw that I'd gotten a Move Up offer (you must be opted in for emails to get these) and it offered us to bid to move to a veranda for as little as $200 total, or $100 per person. The guarantee veranda is currently $400 total above our amazing oceanview deal if we just had paid the price on the website, so I did bid! We have never been to Alaska, and even if the balcony is on the wrong side of the ship for our direction, I still think we will enjoy it.
Wish me luck! I'll try to remember to come back to post what happens later to let you know.