Updated: May 11, 2018
Here is a case when we are unable to change the original duration field once the schedule has been scheduled. This is because the activity has already been started. Let's get into detail explanation:
The Start and Finish Dates which we see at the time of preparing a schedule will be locked as planned start and Planned finish respectively. So, any change to the Original Duration can be seen with a change in the Planned Finish/Finish.
When a Task has not started:
Planned Dates and Start,Finish Dates are linked to each other
Lets see with an Example: Where Planned dates are same as the Start/Finish Dates
When a Task is in Progress:
The Planned Start Date Remains Unchanged if the Actual Start Date is different from the Original Planned Date.
Here in the example, lets status the schedule with actual date of Activity 1 as "26-Dec-14" and schedule(F9) it. you will observe the following changes in the schedule
Planned Start d