Cannot Migrate Contract

I have three contracts with Cell C. One of these, a postpaid voice contract, expired a few months ago and has been running on a month-to-month basis. Today I wanted to upgrade this contract and migrate the number to a smartdata topup data contract with the Samsung Tab E (having waited weeks for the store to get stock). Not possible says the staff at the Hatfield branch, I cannot migrate from an open line voice contract to a topup data contract. I will have to cancel the current contract, give 30 days notice and then take out a new contract in November (...if they have stock). At this stage it would be less painful for me to port my number to a different network and get the device on a better quality network.