Data and minutes I bought forfeited

I had a contract with MTN and due to some reasons which I don't understand, my contract upgrade was declined. I then decided to be on prepaid. When I was in the process to migrate, no one informed me that if I recharge with my airtime or data, when the process is done I will forfeit my balances. I had data and minutes which were not part of the contract package, that I bought with my own money, now it's gone. The consultants are telling me it's there on my contract, which contract are they talking about, because they are supposed to make sure I understand their policies over the phone.