HIGHER COST FOR AN UPGRADE THAN FOR A NEW CONTRACT - WHY?

One would think that Vodacom would try to retain their existing clients [I have been a Vodacom client for more than 29 years] instead of forcing them to cancel their contracts and maybe look at other service providers. My wife and I have 3 Vodacom numbers [2 phones and 1 data only] on a 2-year contract. The contract expires now. I want to upgrade the 2 phones on a SIM only basis. On-line Vodacom offer a “RED Core 3GB 100min Top up” for R 229 PM x 24 months. However, this is for a new contract. If I want the same deal [RED Core 3GB 100min Top up] on an upgrade the price increases to R 309 PM x 24 months. IT IS THE SAME DEAL BUT 35% MORE EXPENSIVE. WHY? I tried to get the answer from Vodacom via Facebook messenger but so far no one has phoned me back, as they had promised.