Over charging

<p>I recently upgraded my cellphone contract with Vodacom after receiving an sms to say I could upgrade my contract 3 months before the contrct expires, I have now received my first bill after upgrading and found it was double the contract amount, I took it as a payment for my old contract and new contract due to upgrading after the 1st of the month, I have now received my second bill after upgrading and still I am still being charged for both contracts, I contacted vodacom customer care to be told by the consultant and her manager that due to me upgrading my contract before the official end date I am liable to continue paying for my old phone until end of May, why is the option of upgrading offered before the contract end date if this is the case? I have upgraded contracts in the past and never had this experience. I find the service and feedback from Vodacom to be appauling.</p>