Upgrade ****

I want to do my upgrade which my due date is in Dec. Go to the store they told me no I can't do the upgrade 4 months before the due date. They can help me but then I must pay the 4 months prorata payments. So, I said no its fine im not gonna do it. Then I Fone the customer care line as I do before and tell them I want to do my upgrade as I explain the lady what the store told me so the lady said no its fine we can help you we have the authority to bypass that on their side then you don't have to pay any prorata money So I said Ohk let's do it then, do the upgrade receive my fone everything sorted. But now I recieved my new statement and the 4 months prorata instalments is on the statement that must pay. I refuse to pay that money because thats not what the lady told me. I emailed MTN explain to them but they don't come back to me. So, what's out for that people. I told them when you fone them they said all calls are recorded so go back to the recordings and listen to what the lady told me.