If you have upgrade your plan (swich to another plan or upgrade your trading limit) during the last month, the difference between your old subscription and your new subscription is payed the following month.
Let's take the example of a subscription update from the Gold plan to the Platinum Plan on November 16th. On December 1st, your will have to pay 129 euros. This amount corresponds to the payment of the Platinum subscription for the coming month (December) plus the difference due to the use of the Platinum subscription during the last 15 days of November.
Hence, we have 99 + 0,5 x (99 - 39) = 129 euros.