UNAUTHORIZED DEBIT ORDER CHANGES

I hold a policy with Dialdirect and the debit date for the premium is the 27th of each month unless this falls on a weekend. and or a Public Holiday. Today (23 rd August) I've just received an SMS from them advising me that they will be debitting my account three (3) days before the debit date. I called and spoke to an agent named Anele and tried unsuccessfully to get the reason why they'd be doing this; especially seeing that the 27th doesn't fall on a weekend or a public holiday. My argument is that I never authorized them to debit my account as and when they deem fit and should the debit order bounce due to insufficient funds, who is going to pay for that. Anele assured me that they would pay . Really ? I wish I was that gullible. That is not even the issue, the issue is why are they reneging from the agreed debit- date without any consent from me. I really do not understand because I have never even defaulted on my payments, so I really need to know why is this supposed to happen !! If I'd known that by taking a policy with them I was giving them permission to do as they please with my bank account, I would have never chosen them, definitely not.??