how can I set Billing date field in Subscription new Line item when the subscription is Amende.

Summary:
The subscriptions are created from Order management, and In SMC, there is a product line Field name "BillingDate" which has four LOV 1. Period Start, 2. Period end, 3. Day, 4. Offset. Initially before Amendment, We are setting "BillingDate" field to "Offset" (which gives a trial period of x no. of days) for line items.

But when we amend a subscription, a new product line is created. We see the "BillingDate" field in new item line is set to "Offset" only. Our requirement is to set the Billing date field to "period start" only for new line after amendment. We want it to be set by default and not manually.