On store and my level no.
On normal contracts (not top up) we can do a number change. Think the charge is about R250.00, but then the old number is not retained on prepaid. Or you can cancel the contract and request to retain the number on prepaid, but then you need to pay the cancellation quote. The third option is to port the contract number to prepaid option with the other networks, then our system will automatically assign a new number to the contract for billing purposes. However, some of the other networks dont like porting contract numbers. They want you to convert to prepaid first.
I think VodacomData has a way of doing what you are asking though. But from my and store front side, no, we cant do it.