Cell C Invoice

My current contract expires on the 12th June 2019. I just received an invoice with previous discounts removed for the remainder of the month, despite having taken out an upgrade with them. This equates to an additional R2500 on my bill. Their T's and C's are as follows: If you don't upgrade or cancel your contract, it will automatically continue on a month-to-month basis; any discounts & promotional value will no longer be applicable. Since I have taken out an upgrade I should not be penalised and discounts should apply based on my new contract. The accounts department is not willing or able to help me. This is not acceptable.