Upgrade of Contract Declined

I have a contract that is eligible for an upgrade with MTN. When I visited the local MTN office I was told that due to a new \credit limit"I am unable to upgrade my contract as my account is exceeding the current limit.<br> On my current contract I am paying more that R400 pm and want to take up a new contract with them for R79pm which will result in a saving of R300 pm. Now that to me make more sence as I will be paying less every month and should be within my so called credit limit with MTN"