Charged for two phones after upgrading

I was sent the customary text from Vodacom I month 21 of a 24 month contract informing new I was eligible for an upgrade. I obliged and upgraded to a more expensive contract (I didn't take out a new contract, only upgraded my existing one).

Unfortunately I was not informed that I would still be required to pay for my old phone for the remaining 3 months.

I have spoken to a lawyer who has informed me that this violates the Consumer Protection Act as the upgrade amounted to a modification of my previous contract and therefore the terms of the previous contract are voided by me signing a new contract. The terms of the new contract should now take precedence.

Could somebody at Vodacom please tell me why this is happening and how this is legal?