"Line Defaults" tab P42101

I'd like to get an understanding of how the Line Defaults tab works when adding and maintaining sales orders through P42101.  I've tried doing some research through learnjde but haven't been able to find what I need.

Our situation mainly concerns maintenance to existing orders.  If we have an order with lines on different branch plants, we may want to change the pick date for a specific branch plant so that the pick date for every line for branch plant A is on a specific date.  We've tried to set the Branch Plant, Scheduled Pick and Promised Delivery Date on the Line Defaults tab but it did not update the Sales Order detail lines.  Does this function only work when entering new sales orders?  If so, is there a way to update the dates for a specific