You can make a netzclub top up with any amount that you wish to retain the old 200MB tariff. Read the answer to the question: "Wie lade ich per Überweisung Guthaben auf?" in the netzclub FAQ list for more details about how to make bank payments.
https://www.netzclub.net/faq.5.html
Approximately one week after a bank payment (Überweisung) has been made, Netclub will automatically send a top up confirmation SMS with the new credit balance.