Dont believe that you can change your debit order they will take a double debit order

Asked on the 12th of April (month ahead) to move my debit order for the 1st of each month to the 10th as my salary date has changed. They said it is fine but the debit order will still go off on the 1st of May for the month of May but the debit order for June will go off on the 10th of May too. I explained to the lady that I do not earn enough money for a double debit order and I wont have funds in my account for the 1st of May. I said to her I do not understand as I pay a month ahead for my medical aid now I need to pay two months in advance? Discovery could not meet my requirements, I have been a member for years. I will not recommend them anymore and they have lost a client. I cancelled my medical aid due to this as they do not have their members back to just move a debit order a few days later without taking a double debit order. On their website it says you have the option to change your debit order but in truth you don't as they will arrange a double debit order that will be two months ahead