Upgrade charges

As notified by Vodacom, I was due for an upgrade (this will be month 22 of 24 into my contract). I subsequently went in store to proceed with my upgrade which was completed and the service assisstant advised that I'm laible for the \old handset"payment for the remaining two months as well as the payment of the new device. I would like Vodacom to advise whether this in fact standard practice to continue charging me for the old device after an upgrade?? """