Sim Swop - Vodacom

I would like to know what is the difference between a Prepaid SIM card and a Contract SIM card. Recently my cell phone was ****** which I had to replace and of course my cell number along with the card. I bought a new SIM card and asked Vodacom to do a SIM swop for me so that I could start using my new phone. Gave them the card number only to be told that I need to go to a Vodacom shop, buy and CONTACT sim card and only then can it be activated. Hence my question, what is the difference between the two cards? So basically their information on their site is FALSE (below): as there they say you can use ANY UNACTIVATED sim card:

"SIM Swap is the process of replacing your existing SIM with a new SIM and moving your existing number, allowances etc. to a new SIM. Please note: Effective from the 11th January 2017 you can use any unactivated SIM card to perform a SIM Swap.Apr 12, 2018"