When you perform a change ABO action, the change is applied to the Asset Term. Fee Amount of billing schedules is calculated based on the Asset Term.

You can also calculate the fee amount based on the initially defined contract term. Consider the scenario: Your contract term is from 1/1/2019 to 31/12/2019. After 6 months, you want to provide a 50% discount. On 6/30/2019, you perform an ABO change action and reduce the price of the asset. Since the discount is applied from 6/30/2019, the asset start date changes to 6/30/2019 and the discount is applied to the new asset term. The discount is applicable to the last 6 months only. 

To apply the discount on the entire contract term instead of the modified asset term,  select Generate for Current Contract in the Simple Discount BS Generation custom setting under Billing System Properties. 

The discount is now applied to the entire contract term. New billing schedules are created to reflect the changed price and all the existing billing schedules are superseded.