The date for the next payment is based on if the customer has credits or is in debt.
Customer has credits
The customer is on credit when the amount of money paid for the ‘old’ subscription, is bigger than the amount that has to be paid for the ‘new’ subscription.
The switch takes place and the recurring payment of the ‘new’subscription will be extended. The number of days of extension is based on the credit value.
Example 1
On 6 July a user subscribes to a monthly subscription of € 20 / month (€ 0,67 / day). That same day he decides to switch to the weekly subscription of € 15 (€ 2,14 / day).
The user therefore did not take advantage of 29 days of the monthly subscription which was € 0,67 per day, meaning he has 29 days * € 0,67 = € 19,43 on credit.
Since the ‘new’ subscription costs € 15 / week, the user does not have to pay anything at the time of the switch, and the next payment date will be in 2 days (19,43 – 15) / 2,14 ~ 2 days, on 8 July.
Example 2
A user subscribes to a monthly subscription of € 20 / month (€ 0,67 / day). After 15 days he decides to switch to the weekly subscription of € 15 / week (€ 2,14 / day).
The user did not take advantage of 15 days of the monthly subscription which costs € 0,66 per day, therefore the user has a credit of € 15 * € 0,67 = € 10,05.
The ‘new’ subscription costs € 15. At the time of the switch he will pay the sum of: € 15 – € 10,5 = € 4,95 and will renew at full price after a week.
Customer in debt
He will pay the difference in price and the ‘new’ recurring fee, on the expiration date of the new subscription.