Vodacom changed my contract agreement

I have a vodacom top up contract , R99 pm for a 2gig top up.<br> Last month Vodacom charged me R3908.68. When querying this I was told that my contract was converted to a regular anytime contract because I was due for an upgrade. My upgrade is only in September and I refuse to accept that this bill is legitimate as I'm contracted for a top up contract.<br> I have no positive response since querying