how to change schedule ship date after booking

 After booking a sales order, I need to split the lines as quantities are being shipped partially.

But the problem is after booking, I am not allowed to change most of the things in the sales order. For example, because partial shipment, I need to change schedule ship date to a future date for one of the lines. But it is disabled.

Do processing constraint, add more constraints rather than allowing to change?

What are the options I have?