Add ability to change iteration length of future sprints, without changing completed sprints.

On my current project we are going to change the iteration length. On past projects, we have also changed iteration lengths partway through. It would be nice if we could change our iteration length for future iterations without affecting the iterations that were completed with a different iteration length. In getsatisfaction, others have asked about this under "Is there a way to change iteration length for just one iteration within a release?" and "Change Iteration Length Mid Project". "How can I set the start date of an iteration?" is also related to iteration length.