I have marked nonworking day as working day(Saturday from non working to working) in my base Calendar(standard calendar) but this change are not getting reflected in my task duration calculation for the existing tasks of the project.
If we are creating any new task or modifying the task date from task page then its reflecting but not automatically for all the existing task.
So how to get it reflected for all the existing task of all the projects?
Try running the Autoschedule feature from Gannt (if for specific Project) , if fine, you can do so for more projects via the Auto Schedule Project Job.
Agreed. Project schedules are not auto-rescheduled like they are when opened in MS Project.
Autoschedule will take into account any change in the factors affecting the schedule.