To allow moving existing OrderItem records to a new Order record in a master-detail relationship:
Option B: Select the Allow reparenting option on the master-detail relationship.
Steps:
Go to the master-detail field on OrderItem.
Edit the field and select "Allow reparenting".
Reference:
"To allow reparenting of the detail records, select 'Allow reparenting' in the master-detail relationship field definition."
- Salesforce Help: Define Master-Detail Relationships
Why Other Options Are Incorrect:
Option A: A junction object is unnecessary since the relationship already exists.
Option C: Changing to an external lookup would not solve the problem and may introduce issues.
Option D: Adding without sharing does not affect the ability to reparent records in a master-detail relationship.