Vodacom at it once again

In December 2015 Vodacom Upgraded my Data LIne from a open Line to a Top Up in order for me not to get theses Surprises anymore when the account comes, I would pay a fix amount, and when the bundle is up I would just buy a new one. It worked for the first couple of months, then come my latest Debit order and its almost dubble, when I call them they said its only going onto Top Up from the First of April, so they **** to me in December, I hope Cell C still has the contract Buy out option because I am done with Vodacom...