Usually, we cannot change the parent-subsidiary once set. However, NetSuite has a feature to specify a period during which authorized users can modify the subsidiary hierarchy. The maximum number of days is 30. You must accept the license agreement before you can modify your subsidiary hierarchy.
Once the feature is enabled users cannot assign the existing parent-subsidiary as a child of a new subsidiary because it does not have a Subsubsidiary of field. When creating a new subsidiary, the system requires a value for the Subsubsidiary of field.
Go to Setup > Company > General Preferences and add a date in the field ALLOW SUBSIDIARY HIERARCHY TO BE MODIFIED [MAX: 30 DAYS] and save to enable parent edit feature
Additional notes, ref