Billing frequency changed from monthly to annual

As an example let's use a subscription with billing date - 7^th of the month. On the 14^th of May customer decides to change from monthly to annual billing frequency. Going forward subscription will have the new billing cycle day for subscription. New billing cycle date is 14^th (day when frequency change was done) and first billing period with new cycle will be from 14/05/2019 till 14/05/2020.

In period 07/05/2019 - 07/06/2019 are 31 days. Monthly fee is 62.93, price for 1 day = UnitPrice/(days count in billing cycle) = 62.93/31=2.03.

In period 07/05/2019 – 14/05/2019 are 7 days, UnitPrice for 7 days = 7*2.03 = 14.21.

In period 07/05/2019 - 07/06/2019 used quantity is 102, TotalPrice = 102*14.21 =1 449.42.

In Invoice 3 - TotalPrice=6 418.86. 

Due to billing frequency change from monthly to annual on 14th of May, then for period 14/05/2019 – 07/06/2019 customer has not used subscription. As such monthly cycle and Invoice 4 contains Correction for period 14/05/2019 – 07/06/2019 for unused days = 1 449.42 - 6 418.86 =-4 969.44.

In Invoice 4, line with ChargeType = Purchase Fee is first fee after frequency change. UnitPrice = (monthly price) * (month count in period 14/05/2019 - 2020-05-14) = 62.93*12 = 755.16.