Change of debit order date without my authorizatio

After months of fighting, I managed to sort out the problem on my account myself. I did my contract upgrade on 28 December 2014. My debit order date was changed from the 29th of every month to the 28th of every month WITHOUT my knowledge or my authorization. I only realized that when I got an sms from my bank stating that a debit order for my Cell C account was reversed due to no funds in my account and was penalized R115 for a mistake that was not mine. I phoned cust service where they said the date was changed at upgrade department. But I did not change my debit date, nor did they tell me the date has changed. Now I have to go and deposit the money into their account and also have a record of non payment. I wonder how long it will take for Cell C to get their act together and to be able to give better service, as their service is still PATHETIC. I am ASHAMED to say that I am a Cell C customer and regret upgrading my contract and not just letting it run out and going to MTN. I demand that the R115 be paid back into my bank account as this is not my mistake. And I refuse to take responsibility for that.