Early upgrade, double invoice.

I was shown on my account online that I qualified for an upgrade. So I phoned a store and organised the phone that I wanted to upgrade to. I went to the store when they told me that they had stock of the phone I wanted. When I received my invoice, I was charged for the old phone and the new phone and I phoned in to query, they told me that that is now how they are billing clients. I phoned again and spoke to another person as I couldn't believe a company would change their T's and C's without communicating to clients. I now have to pay for 2 phones for the next couple of months. This is ridiculous.