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 how this is legal? Also, please do not tell me that it's because the term of my contract is 24 months. I have already been told this. My last upgrade was 24 months ago so I've already paid 24 installments for my old phone. Paying a further 3 amounts to 27 installments.