Cell C contract cancellation issues

What a joke!!! My Cell C contract expires on 11 September 2018. I don't use my phone much, so decided to convert to a prepaid option. As a result, I contacted Cell C well in advance to let them know of my decision to do so...thinking that they would be technologically advanced enough to automatically convert my contract to prepaid on the given date (11 September). I was told that this is not possible and MUST PHONE BACK ON 11 AUGUST 2018. I set a reminder on my phone to do so and duly called them a couple of minutes ago to cancel the contract and convert to prepaid option, effective 11 September, only to be told that they don't do the contact to prepaid swops over a weekend and I Should phone back on Monday...initially, when I tried to get them to flag the change, I was told I would have to pay a fee for the service...are they now going to penalise me for being late when I phone on Monday...one can't win?