Work Order Start and End end date changing

Scenario:

Work Order - 1234

FG - XYZ

Scheduled Start Date - 01-Apr-2018

Scheduled Completion Date - 30-Apr-2018

Operation level:

- Op-10: 01-Apr-2018 to 10-Apr-2018

- Op-20: 11-Apr-2018 to 20-Apr-2018

- Op-30: 21-Apr-2018 to 30-Apr-2018

At this stage, if we move the first operation by 10 days and new Op-10 start date becomes 11-Apr-2018, it is changing the work order header level start date also.

Issue here is - we don't want the work order header level start date to move in spite of moving the op-10 start date by 10 days.

How can we prevent the header date from changing.

Regards,

Pranit.