The tasks are not rescheduled when you change the project to schedule from a finish date.
When you choose to schedule a project from a finish date, you should examine the effects on any tasks that are already in the project. If you initially scheduled your project from a start date, the tasks were created with an As Soon As Possible (ASAP) constraint by default. When you change your project to schedule from a finish date, all new tasks will be created with an As Late As Possible (ALAP) constraint by default. However, the original tasks still have ASAP constraints that can affect when the tasks are scheduled as you enter actual start and finish dates. This can create unnecessary negative slack on the tasks if the tasks start late.
The amount of slack in your schedule tells you how much you can delay tasks before other tasks or the project finish date is affected. Total slack is the amount of time that a task can slip before it delays the project finish date. When the total slack is negative, the task duration is too long for its successor to begin on the date required by a constraint.
Examine the task constraints for all your tasks, and change ASAP to ALAP wherever appropriate.
Change task constraints from ASAP to ALAP
On the View menu, click Gantt Chart.
Click the field to the right of where you want to insert the new field. For example, click the Task Name field to insert the field immediately to the left of the task's name.
On the Insert menu, click Column.
In the Field name box, click Constraint Type, and then click Best Fit.
To identify any tasks that have ASAP constraints and, if appropriate, change them to ALAP constraints, click AutoFilter .
Click the AutoFilter arrow in the Constraint Type field heading, and then click As Soon As Possible.
Click the arrow in the Constraint Type field of the first task displayed, and then click As Late As Possible.
Point to the lower-right corner of the field. When the pointer changes to a plus sign, drag down to set the fields for all the tasks to As Late As Possible.