They changed my open contract to Prepaid....WHY???

I recently renewed my cellphone contract with Telkom and as part of the upgrade received a Samsung Galaxy A72 phone. Problem is that instead of them upgrading my contract as is, they went and changed it to a Top Up only contract, which now means that as soon as my included free minutes gets used up, I am now greeted with an annoying voice message telling me that I cannot make a call as I don't have any minutes left. I ALWAYS had an Open contract with Telkom, which meant that I could phone and then get billed that amount at the end of the month. Why did they go and change my contract to a prepaid? This is not what I wanted. I wanted my original contract, moved over with the new phone. I spent all day trying to get a human being on the line, and eventually went to the Telkom shop. They now want to bill me for the entire cost of the phone and give me a new Sim. This is not what I want. I just want things to return to the way it was where I could phone on a normal contract, as I've been able to for the last few years since joining Telkom. Please sort this out.