But we did recently cruise on the Dream last weekend (not in concierge) and kept checking for cabanas every day. Finally saw cabana 21 open up and booked it immediately.
when we got on board, we let customer service know that we would be willing to switch to a smaller cabana if there was a cancellation or someone wanted to switch with us.
the next day, the concierge called us and said they switched us to the smaller cabana that we really wanted and another guest got the large one they wanted. So it worked out well for everyone.