Telkom cancelled my airtime for no reason

I upgraded my contract on Telkom while keeping my number and have found that my airtime balance was lost in the process. This was my prepaid airtime and had nothing to do with the contract. On enquiry the call centre said that I forfeited my airtime by upgrading the contract. At no stage was I informed by the consultant that I will lose all my airtime by upgrading. This is unacceptable!