Did you see the patterns talk at the Plone Conference in Arnhem 2012? Here are the slides that were used for the demonstration.View slides
Now the designer can validate and iterate a rich prototype with both the business stakeholders and the developers. All possibilities and impossibilities will become apparent before anything is built. This will prevent a lot of change requests and headaches during the development phase.
For changes to an existing design, the process will look more or less the same, except everything will likely happen in shorter iterations and more in parallel.