The “Billing Plan Item” assignment block is no more available in CRM by design until the note 1405086. After this only tab ‘conditions’ of “Billing Plan Item” assignment can only be displayed – not edited.
The standard approach is to have the conditions of the individual periods not in edit mode but in display mode only due to the fact that changing conditions for one billing request item might (due to grouping set up for the billing plan) have effect not only for the selected period but also for other periods.
Adjusting conditions for non billed periods would be possible by using so-called structure conditions on the main item and maintain them on the main item. You can find details concerning this in notes 1046336 and 1385298.