Migrated for Contract to prepaid: wiped-out 14G of data and airtime that I purchased

I have an MTN customer for more than 25 years. Last month I decided to migrate to pays as you go since I was not ready to upgrade my contract. On Tuesday 27 August, MTN uploaded my minutes as usual. They then reverse the minutes and thereafter wiped 14gig of Data that my employer provided for me so that I can work form home. They also wiped some of the a R180 airtime I had purchase. When I called to complain, the response I got is "Sorry Ma'am, that is how MTN operate when migrating customers from contract to prepaid. As much as this was NOT explained to me initially, it does NOT make it right for MTN to have such policy. Why does MTN see it fit to wipe out customers' data and airtime that they purchased with their own money. If the system cannot reverse what they did, they can send me a voucher with data and airtime to reimburse what they stole from me.