CANNOT CHANGE DEBIT ORDER DATE

I have been a loyal customer of Cell C for a few years already and now I have resigned in my current position.<br> When you resign, you get paid on the last day of the month. Naturally, I have to change my debit order date as it was on the 27th. I called on the 11th of April to change the debit order date and I was told that I cannot. The system does not allow it as it is already the 11th. I signed my new contract on the 10th and only resigned then. How and when was I supposed to let them know that I need my debit order changed before hand? I need assistance in this regard please?