You can easily switch to another rate plan free of charge if you already have a Pay&Go card, subscription or Full Control (What is Full Control?).
Want to know the rates of your current plan? Consult your rates. To know the name of your current Pay&Go rate plan, type #122# and press the call button.
Note: 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.