Vodacom is sharks!

My contract is due for an upgrade - I've selected the device and the contract but when I try to upgrade then I get an error message that the deal is not available for this contract. Why not? Why is the deal only available on new deals and not upgrades? Why is the deal more expensive if it is an upgrade? The call centre can't help, they don't know what is hapening and they are unwilling to cancel the contract even if it is at the end of its term.