Changing the subscription can be done during the subscription. You can switch subscriptions from your camp settings.
Example:
Purchase of a subscription to 60 € excluding VAT per month on 01/09/2016
Change of subscription on 15/09/2016 for a subscription to 100 € per month
The 50% of the non-consumed subscription from 15/09 to 30/09 will be deducted from the new subscription with a value of 100 € excluding VAT per month.
Calculation method: 60 € / 2 = 30 € excluding VAT
New subscription: 100 € - 30 € = 70 € excluding VAT
You will pay online € 70 excl. VAT for your subscription from 15/09/2016 to 15/10/2016