This feature is available in Catalyst, Enterprise, and Ultimate editions.
There are several ways to change a project schedule. Choose the one that best fits your scenario.
If no workitems are assigned to a sprint/iteration in the assigned schedule, you can change the schedule following the instructions in this article: Assigning Iteration Schedules to Projects. Note that your project role must be set to "Project Admin" or higher.
If work has not been closed or effort has not been tracked, you may be able to remove the stories from the sprint, and then change the schedule.
If the schedule can be edited, the "Clear" link displays. If not, the schedule does not meet the requirements for change. This probably means that work is already assigned to a sprint/iteration.
If a team has an established sprint history in a project (meaning you have logged work effort or closed workitems in closed sprints), the best approach is to create a new project that uses the new sprint schedule, and then move all of your open backlog items to the new project. You can create programs to link the old/new version of each project for historical reporting purposes.
For example, if you start with a project tree such as:
- Organization -- Project A (Schedule A) -- Project B (Schedule B) -- Project C (Schedule C)
First, create a new set of parallel projects that are in the new schedule:
- Organization (New Schedule) -- Project A (Schedule A) -- New Project A (New Schedule) -- Project B (Schedule B) -- New Project B (New Schedule) -- Project C (Schedule C) -- New Project C (New Schedule)
Then, move all of the open backlog items in each project to the new version of that project.
Create the programs (e.g., Project A Program (contains both 'Project A' and 'New Project A')).
At this point you can close the old project to prevent confusion, and just use the Show Closed Projects option when you want to view historical information. If you want to leave them open, you can rename the old project, to clearly specify they are the old versions.