Do you already have a subscription or Pay&Go card? In that case you can easily switch to another rate plan free of charge. Consult the rates of your current rate plan. Do you have a Pay&Go card and you don't know your rate plan? Type #122# and press the call button.
Did you choose a mobile phone when you took out a subscription? Usually, you can change your rate plan afterwards, but you can't change your DataPhone option.