This article discusses the three dimensions of user stories: the backlog order, the complexity (or effort) and the business value. These dimensions are flexible until the sprint commitment, but right before it, you should have estimated the complexity and the business value for the top of the product backlog, which means that the product backlog has been re-ordered. The sprint commitment can then occur: the team pulls the stories from the top of the product backlog into the sprint backlog, negotiate the order and amount with the PO. After the commitment, complexity and business value of a story should not be change to keep the metrics meaningful.
