LOST OVER R500 - BALANCE ON MY PHONE

I have upgraded with MTN on Saturday with a balance of more than R 500 left on my contract. MTN couldnt give me the same package as per my old contract, the changed the money for airtime to minutes available. No problem....BUT they took my balance on my old contract and did not transfer it to the new upgrade. Isn't it theft. I pay upfront for my airtime, how can I just loose my balance on airtime? <br> <br> The Branch at Kolonade is trying to help me getting it back but MTN says I may cancel my contract if I want to but they dont transfer money