Master/Details change of the master linked number in details is not happening

Sometimes, after we make a sell order, customers change their purchase orders to 2 or 3 purchase orders. Then we have to create one or two more orders and distribute the details to the new orders according to their new purchase orders.
I use a form with Multiple records as details form, with a where clause linked to the order.
When I change the linked order number on the details form, two events are not happening, the select box is not tagged automatically (which happens on the other fields in the detail), and when I “Save Selected” after tagging the selected box, then the original order number returns in the select field.
I am out of ideas, so any help will be appreciated.