Issue converting contract to prepaid

Hi, I am having an issue with Vodacom - on the 3rd of May I phoned Customer Retention ( ********** ) and asked them to cancel my expiring contract and convert my number to prepaid. The lady assured me it will be done and that NO further costs will be subtracted from my banking account. Yet on the 1st of June the debit order went off and if I log into www.vodacom.co.za it says that I am still on contract.