Upgrade online - contract changes , Large invoice

I have atop-up contract. mobile phone in my name , this contract is a top-up contract for my son. He has had this contract for many years and the contarct type has never changed. We decided in January to upgrade the contract, to get a new phone as it was up for an upgrade. Our contract was not changed, just upgraded. We still need a top-up contract. I receieved a shock when I look at the contract online in February 2016 only to find that it is more than R8000.00. I phoned Vodacom and they told me that I selected a normal contract. I told them that this phone was always on a Smart S top-up contract. They now told me that I selected the wrong contarct type and that it must run for 3 months. There was no indication that my contract type will change when I selected it online.I spoke to a Supervisor after no luck with the normal support staff. He logged an issue with their back office and I'm still waiting for a reply. I want this contract changed immediately and changed to a top-up contract and I will not pay for this as I was not warned that my contract will change. I will evenually move all 10 my contracts if this is not resolved in a decent way.