Debit Order change just because it December

They want to change my debit order that i have sign for the first of each month. When you phone in the person in account say she will log it ,then i ask if it not have change what can we do and how will do change at ITC/ Experian and paid the unsuccessful debit order fees at my bank now the person said i am make Sumsion's it will not be change. if you already send sms out then you have made the change, I have at many other vender that send out sms like this and you phone the and tell them not to change they still change it. There is a reason why you have select a day of teh mont to do the debit orders.