Vodacom wont let me upgrade 2 a lower cost package

i have been trying to upgrade my contract for 3 weeks now with no luck and no service. I went to the Vodacom in North Gate and requested to upgrade my contract. im currently paying about R600pm on my current contract excluding my topups i do in the month. Roughly i pay about R1400pm in total for this contract. I wanted to upgrade to the Sony Xperia Z3 as it is advertised for R470pm which is R130pm cheaper then what im currently paying excluding my topups. But i was told that i can not take a lower pm contract....*** that just dose not make sense to me. im am now being forested to keep paying FOR the higher contract then what i want, till the contract is over in 3 month. So disappointing that one spends all this money every month for a service and get nothing...Im still waiting for the Manager Brandon @ North gate to get back to me with answers..... shocking VODACOM SHOCKING!!