I've changed my plan from Yorn 10GB to Yorn 1GB at a Vodafone store and they said that you will receive a message which you should confirm saying "SIM." I replied the message with SIM but it said that we can not process your transaction since there is no enough balance. However, I paid 6 euros that inclue 2 weeks. Now, I will have to pay 6 Euros again although I just should pay 2,75.