bad service not willing to help

I have taken up a contract on the 18th July where I am paying 179 pm and 3 days later on the 21st July the same phone with the same bundle structure is going on for 129pm, when I phoned the call centre to find out if they can change my premium to the 129pm as my 1st debit has not yet gone off but the rude call centre agent just said \sorry we cannot change the premium for you"I don't see why I should pay an extra 50 rand a month for the exact same contract "