Debit Order dates being dictated!

I just had the WORST experience for just wanting to change my debit order date. I called in last month to change my banking details for a successful debit every month, I was told to call back at the beginning of this month because the debit order instruction has already been sent. I call back today, spent 15 minutes on the call with the agent going back and forth on why he cannot change my debit order date. The more i explain that i can even send him my employment contract stating that my salary date is now the 28th, he is pushing back saying my previous salary transfers are on the 26th and he will have to use that date regardless of what my employment contract says. I got so upset saying that why is it being made so difficult for me to change my date accordingly to ensure that no attempt on my debit order goes unsuccessful. Why would they not welcome such an act????

NOT great FNB, really! Been banking with you for more than 10 years... NOT great!