Upgrade conditions changed without notice

I was contacted by Vodacom with regards to an upgrade on one of numbers, I queried this with operator who contacted me as I was not sure about this, but they were adament that I was, I then proceeded with the upgrade. When I received my monthly statement I saw I was paying two device installments. I queried this with customer care and was told that this was correct. I asked how is it possible, they explained that the rules of the upgrades had changed and you could now upgrade after 18 months and not the normal 22 months that I was accustomed to. This was not shared with me during the upgrade call, if I was aware of it I would have not proceeded with it. I am still waiting for someone to contact me to resolve this issue. I feel that I have been grossly mislead!!!!