UPGRAGE: CHARGE FOR OLD DEVICE

In December I went to mall at carnival to upgrade as my upgrade date was 06 December 2015. I now receive my January statement and lo behold, I am being charged for the previous device!!! This is not my first upgrade with Vodacom and I have never experienced such a practice before. I called Vodacom customer care and I have been send from pillar to post with each department passing the bark to the other. I have had problems with Vodacom before (i.e. forgetting to inform me of upgrades and being billed not in accordance with contract). Despite that I have stuck around giving it a benefit of the doubt, but after this latest stunt I will not be using them anymore. I would like for the charge to be removed or alternatively let me go back to my old contract and on expiration to apply for a new one (definitely not with Vodacom after this debacle).