Airtime Lost

I would like to change from contract to prepaid but MTN says they will not move my airtime to prepaid. I have R4000 plus airtime and would not like to lose it. Vodacom said they would move it over if I were there customer.<br> They cunningly handed me a form that if I signed would amend the original contract to accept that I would loose my airtime . I did not sign this form. Why can they not convert my airtime , I have paid for it? If they cant then they should pay me my money back.