Changing my vehicle debit order

[Tella1996]

Good day,

I was at my branch this morning, I called Standard bank VAF.
I asked for them to change my Vehicle debit order date to the first of each month because my current date is the 27th of each month and we normally get paid on the 28th of each month.
Now the person I talked to said that they can't help me in changing the date because then there would be an increase in my monthly installment??
How is this possible. I really do not understand.
Help would be appreciated

Because now the payment might bounce..