Since 2015 is closed and the plan is to execute mid-year. Have they entered a Fiscal 2016 that has correct period end dates. Changing the periods at mid year will require a Financial reconcile (which they would be warned of when they make the change). Also required would be a rebuild of the Fixed asset calendar to sync to the new dates. As for Project and your third parties, unless they have a calendar, they should be OK.
Tim Foster